Project calendar: working days, holidays and why a 5-day task can take 9

A task estimated at 5 working days starts in an Italian office on Wednesday 30 December 2026. It finishes on Thursday 7 January 2027, 9 calendar days later. Four of the days in that window are public holidays or weekend, and the project calendar accounts for them: a plan on a plain Monday-to-Friday week with no holidays entered would have promised Tuesday 5 January.

This guide explains how a project calendar converts durations into dates and who should maintain it. It also covers when one calendar for the whole project is enough and when a person or a supplier needs a resource calendar of their own. The worked example follows a packaging reprint for a pasta maker across the Italian August closure: one missing supplier calendar moves delivery by 5 working days, and one working day of slip in July moves it by 22 calendar days.

What a project calendar is#

A project calendar is the rule a scheduling tool uses to convert working days into dates: the working week and hours, plus exceptions such as public holidays and company closures. Without one, the tool counts days on which nobody works, and every date that depends on them comes out early. In a pilot schedule for the US Department of Homeland Security's Transportation Worker Identification Credential (TWIC) programme, the Government Accountability Office (GAO) found 259 of 302 activities, nearly 86%, on a 7-day calendar that did not account for holidays^[1].

Two details from the same case show the cost. An earlier version of the schedule had several pilot sites finishing the submission of their test data on Christmas Day, and GAO points out that an activity defined as 60 days, about 2 months on a 7-day calendar, may take about 3 months when the people doing it work a 5-day week^[1].

GAO's Schedule Assessment Guide defines calendars by the number of work days per week and the hours available on each work day, with holidays and plant shutdowns as examples of exceptions^[1].

Two terms recur below. A working day is a day the calendar marks as available for work. A calendar day is any date on the wall calendar, weekends included.

Who needs a project calendar, and when#

The project manager or planner builds and maintains the project calendar, and one calendar is enough for a team of 3 or 30 people as long as all of them work the same week in one country.

Headcount matters less than the number of different weeks in the team. The inputs come from other people. Ask the team leads who works part-time, and on which days. Closure dates for Christmas and August come from whoever announces them, often HR or the office manager, and GAO makes resource managers responsible for identifying and assigning the correct calendar to their resources^[1]. Everyone who reads a date also reads the calendar, mostly without knowing it: the sponsor looking at the finish, the client waiting for a delivery.

Add calendars beyond the first one in these cases:

• someone on a critical task works part-time;
• an external supplier closes on dates of its own, like the printer in the worked example below, whose three-week shutdown starts a week before the client's two-week closure;
• the team spans two countries.

Set the calendar before the first duration goes in and before the baseline is saved. Review it each autumn, when next year's closures are announced, and whenever the plan crosses 31 December.

Skip the extra work in two cases. A plan of under 10 working days with no holiday in its window can live on the default Monday-to-Friday week, because the only non-working days it can cross are two weekends, and the default week already skips them. Per-person calendars for a team of 20 cost more than they return: GAO warns that too many calendars may interfere with critical path analysis and that the administration needed to build and maintain many resource calendars may quickly outweigh their benefits^[1].

How working days and holidays turn into dates#

Calendar days = working days + the non-working days that fall between the start and the finish.

A scheduling tool starts each task on the first available working day and counts only working days until the duration runs out. The table runs the same 5-day task from four start dates on an Italian office calendar, Monday to Friday. Article 2 of Italian Law 260 of 27 May 1949 lists New Year's Day, Epiphany (6 January), the Assumption (15 August), Christmas Day and 26 December among the public holidays^[2]. The last row adds a company closure from 24 December to 6 January.

The third row needs two years of holidays. A task that starts in the last two working days of 2026 crosses New Year's Day and Epiphany, and a calendar that holds only the 2026 holidays puts its finish on Tuesday 5 January, two calendar days early.

The table also shows that the stretch belongs to the start date, so it changes every time a predecessor moves. A task planned to start on 18 November takes 7 calendar days; if its predecessor slips until 23 December, the same task takes 21. Our recommendation follows from that: give clients and sponsors dates, and keep durations for the team. The exception is a quote whose start date is not fixed yet, where working days plus the name of the calendar they run on are the only figure you can stand behind. "Five days" means 5 to the person doing the work and 9 to a client who hears it on 30 December.

The calendar also fixes the hours in a working day. A job of 30 hours of effort is 4 working days at 7.5 hours a day and 3.75 at 8 hours (the guide to estimating task durations separates effort from duration). Half days, such as an office that closes at lunchtime on 24 December, are exceptions with reduced hours. Lags follow the same arithmetic, which is why lag in working days and lag in calendar days can land on different dates.

One project calendar or one per resource#

One project calendar is the default; a resource calendar is worth adding only for a person or supplier whose non-working days differ from the project's and whose float is smaller than the working days their closure removes.

The project calendar holds the organisation's week and its exceptions, for example the Italian holidays and an office closure from 9 to 20 August. A resource calendar says when one person or one supplier can work: a designer on Monday to Wednesday, a printer shut for three weeks in August. GAO's guide mentions a testing facility that works 24 hours a day while some staff work four 10-hour days a week^[1]. Microsoft Project warns that telling it to ignore resource calendars may assign work during a resource's non-working time^[3].

Our recommendation: one project calendar, plus one extra calendar per supplier that closes on dates of its own, except when the supplier's tasks have more float than the number of working days its closure removes, in which case a note on the task is enough. In the worked example the printer's shutdown removes 15 working days and its tasks sit on a single chain with zero float. So the printer gets its own calendar, and two calendars cover that plan.

Personal calendars pay off only for part-time people on the critical path.

A team of 8 in one office with one part-time designer on the critical path and one outside supplier therefore needs 3 calendars: the project's, plus one each for the designer and the supplier. A single day of leave for someone on a task with float is easier to catch in a workload check for over-allocation.

A worked example: a packaging reprint across the August closure#

In this constructed example, one missing supplier calendar moves delivery from Wednesday 1 September to Wednesday 8 September 2027, 5 working days later.

An Italian pasta maker is reprinting the 500 g boxes of its fusilli line: 80,000 boxes with a new recycling label. The plan has 5 tasks linked finish-to-start, 18 working days in total. The label wording comes back from the company's regulatory review on Friday 23 July 2027, so artwork approval starts on Monday 26 July. The company works Monday to Friday and closes from Monday 9 to Friday 20 August 2027; Italian public holidays are on its calendar as well. The printer uses the first week of its shutdown for annual press maintenance, so it closes from Monday 2 to Friday 20 August, one week longer than the client. It confirmed those dates by email when the order was placed; the buyer filed the email in the order folder, and nobody copied the dates into the plan. The Assumption falls on Sunday 15 August 2027, so that public holiday removes no working day from either calendar^[2].

Transport needs one rule. The printer palletises each run on its last print day and the pasta maker's haulier collects the pallets that afternoon. Task E, transit and goods-in at the plant, is the client's work and follows the company calendar.

Version 1 puts everything on the company calendar. Proofs arrive on Wednesday 4 August and are signed off before the office closes on Friday 6. Printing then waits for the reopening on Monday 23 August, and delivery falls on Wednesday 1 September.

In version 2, B and D follow the printer's calendar. The printer works on the proofs on Thursday 29 and Friday 30 July, closes for three weeks and finishes them on Wednesday 25 August. Sign-off moves to 26–27 August, printing to 30 August–6 September, and delivery to Wednesday 8 September.

Five working days of proofs now span 28 calendar days. The plan is 18 working days long in both versions, which is 38 calendar days in version 1 and 45 in version 2. Version 1 fails quietly: on Wednesday 4 August the client waits for proofs from a printer that has been closed since Monday 2.

A rejected proof adds 7 working days. If the plates have to be remade after the sign-off of 27 August, the second round of proofs runs from 30 August to 3 September, sign-off moves to 6–7 September and delivery to Friday 17 September.

The closure also changes what a slip costs. Suppose the label wording had been ready earlier and the same 5 tasks were planned to deliver before anyone closed. The print run would then have to end on Friday 30 July, the printer's last day before its shutdown, and counting backwards on the two calendars gives a latest start of Friday 9 July 2027. Here is the same plan started one working day later, on Monday 12 July:

With the later start, the sixth day of printing falls after the shutdown, on Monday 23 August. Delivery moves 22 calendar days.

How to set up a project calendar with holidays, step by step#

Six steps, done once before the first duration is entered and repeated each time the plan crosses a new year.

1. Set the working week and the hours of a day. Monday to Friday, 8:00–13:00 and 14:00–17:00 gives an 8-hour day, and that number converts every estimate given in hours.
2. Add the public holidays for every year the plan touches: a plan from November 2026 to March 2027 needs both years.
3. Enter company closures as named exceptions, such as "Summer closure, 9–20 Aug 2027", so that a reader of the Gantt chart can see why a bar stretches across August.
4. Ask each supplier on the critical path for its closure dates in writing when the order is placed, and copy them into the plan the same day. Give that supplier a calendar of its own: a three-week August shutdown at the printer appears nowhere on the client's calendar.
5. Record part-time patterns as resource calendars.
6. Check two dates by hand. Pick one task that crosses a holiday and one that crosses a closure. Count their finish dates with the formula above, on a printed calendar if needed, and compare them with the dates the tool shows before anyone else reads the plan.

Common mistakes#

GAO describes a common calendar mistake: leaving every activity on the scheduling software's default calendar, which rarely has national holidays defined as exceptions and does not define blackout periods^[1].

• Holidays for one year only. Without 1 and 6 January 2027 in the calendar, the 5-day task from the opening finishes on 5 January, two calendar days before the real finish on the 7th.
• Padding in place of exceptions. A task stretched from 5 to 8 days "because of Christmas" keeps its 8 days when it moves to February, 3 working days too many.
• A 7-day calendar for office work. In the TWIC schedule it covered 259 of 302 activities^[1].
• The client's calendar on the supplier's work, as in version 1 of the example. The inverse also happens when someone types a supplier's three-week shutdown into the project calendar, which then stops the client's own sign-off and goods-in tasks during the week the client's office is still open.
• One calendar per person for a team of 20: 20 calendars to update each time someone books leave.

The limits of a project calendar#

A project calendar marks each day as working or non-working, so a day when half the team is on leave still counts as one full working day.

GAO's guide recognises that productivity is generally low in the US holiday season, from Thanksgiving to New Year's Day, and notes that a schedule risk analysis can handle this with probabilistic calendars, where any day has less than a 100% chance of being a working day^[1]. Our recommendation: mark in the calendar only the days when the office is really closed, and treat the week before the Assumption (15 August) and the days between Christmas and Epiphany as schedule risk in a Monte Carlo schedule risk analysis, with wider pessimistic estimates for the tasks that fall in them.

Some waits ignore the calendar. A supplier lead time quoted as 14 calendar days and starting on Monday 21 December 2026 ends on Sunday 3 January 2027. Entered as 14 working days on the Italian calendar, the same wait ends on Tuesday 12 January and spans 23 calendar days, 9 more than the supplier quoted.

Mixed calendars also make float harder to read, because calendars can affect float calculations^[1]. In the example, one working day of slip before 30 July costs 22 calendar days at delivery, while the same slip in September costs 1 calendar day, or 3 across a weekend. A float figure in working days shows both as the same single day.

How to do it in Planitude#

Choose the calendar when you create the project, because it cannot be edited afterwards. Describe the project to the AI assistant: it proposes the country's public holidays for the plan dates, plus the closures you mention, for you to check in the preview Gantt. A blank project stays on Monday to Friday, 8 hours a day, with no holidays.

Durations and lags count working days on those calendars, and the Gantt chart shades weekends and holidays. Imported files keep their calendars. A task with no calendar of its own takes its resources' calendar when they all share one; the import report counts the tasks whose resources did not.

Supplier and personal calendars have no editor. For the printer, put a "start no earlier than" constraint on task B at the reopening date. It is cautious: B runs 23 to 27 August and delivery moves to 10 September, two days after version 2. The Workload view still checks part-timers against their calendar and availability. Lags have no calendar-day unit.

Frequently asked questions#