Calendar Calculator

the weekday of a date is fixed by the proleptic Gregorian calendar; every other number describes the whole month that date belongs to, not the date itself

date

Any date from 0001-01-01 to 9999-12-31; it decides which month gets described

weekday

The day of the week for that date, counted from Sunday

daysInMonth

The length of that month: 28, 29, 30 or 31 days

firstWeekday

Which weekday the 1st falls on, which decides how the whole month is laid out

weekendDays

How many Saturdays and Sundays the month contains: 8, 9 or 10

isoWeeks

How many different ISO week numbers the month touches: 4, 5 or 6 weeks

Use it when you need one date placed in its month rather than a span between two dates: how a shift pattern lines up, whether the 1st is a Monday, how many weekend days a month leaves you, or which week number a delivery falls in. It answers those from the date alone, so there is no year and month to fill in first. What it does not do is draw the month as a grid, and it does not know about public holidays — weekend days here are always Saturday and Sunday, whatever your country treats as a rest day.

1. 19 September 2026

   1. The month is September 2026, which has 30 days
   2. The 1st of September 2026 is a Tuesday, so the month opens on day 2 of the week
   3. The 30th is a Wednesday, so it closes on day 3 — the month is exactly four weeks and two days long
   4. Saturday and Sunday appear four times each, giving 8 weekend days
   5. The month touches five different ISO week numbers, because it starts mid-week and ends mid-week

   The two weekday numbers at the top and the bottom of the month are what decide whether a four-week roster fits cleanly. A month that opens on a Monday and closes on a Sunday is the only one that does.

2. 31 January 2021 — a month that crosses the ISO year

   1. 1 January 2021 is a Friday, and the ISO week runs Monday to Sunday — so that week's Thursday is 31 December 2020
   2. The whole of 1 January therefore belongs to week 53 of 2020, while 31 January belongs to week 4 of 2021
   3. 31 January 2021 is a Sunday, and the month has 31 days
   4. This month holds five Saturdays and five Sundays, so the weekend days total is 10 rather than the usual 8
   5. It still touches only five different ISO week numbers: 53, 1, 2, 3 and 4

   This is the month that breaks the obvious implementation. Subtracting the first week number from the last gives 4 minus 53, which is not a count of anything. Counting the distinct week numbers a month actually touches is the only version that survives January.

3. 29 February 2024 — a leap February

   1. 2024 is divisible by 4 and not by 100, so February has 29 days
   2. 1 February 2024 and 29 February 2024 are both Thursdays — 28 days apart, and 28 is a multiple of seven
   3. So the first and last weekdays of the month are the same, which only happens in a leap February
   4. Eight weekend days, five ISO weeks

   A common-year February has 28 days and so also opens and closes on the same weekday — but it lands on a Wednesday in 2026 and a Saturday in 2027. The leap year is what lets the whole month stay aligned with the week.

Weekend days are counted as Saturdays plus Sundays, always. Countries with a Friday–Saturday weekend, and anyone whose rest day is Sunday alone, will see a number that does not match their own week — the business days tool is the one that lets you pick a weekend rule. Public holidays are not subtracted either, and there is no built-in list of them: the answer is about the printed calendar, not about which days are actually worked. There is no month grid on this page, because the reference tables are built once at build time and cannot see the date you typed; the seven numbers are what a grid would have told you. Dates are proleptic Gregorian throughout, so the calendar is projected backwards before 1582 without any reform, and the year range is 1 to 9999.

What day of the week is 19 September 2026?

A Saturday. The weekday depends only on the calendar date, so it does not change with the time zone you read the page in: 19 September 2026 is a Saturday in every country. The month it belongs to has 30 days, opens on a Tuesday and closes on a Wednesday.

How many days are in each month?

Seven months have 31 days, four have 30, and February has 28 or 29. The 31-day months are January, March, May, July, August, October and December; the 30-day months are April, June, September and November. February has 29 days in any year divisible by 4, except centuries not divisible by 400 — so 2024 has 29 and 2100 will have 28.

How many weekend days are in a month?

Usually eight, and sometimes nine or ten. A 30-day month spans four weeks and two days, so two weekdays get a fifth appearance; if both are Saturday and Sunday the month has 10 weekend days, which is what January 2021 does. A 31-day month with five Saturdays and five Sundays is the other ten-day case.

How many weeks are in a month?

Four, five or six, depending on how you count. Counting the distinct ISO week numbers a month touches gives five for most months, and six in two cases: a 31-day month starting on a Saturday, or a 30-day month starting on a Sunday. Four needs a February of exactly 28 days starting on a Monday — the 1st is the first day of an ISO week and the 28th is its last.

Why is the first weekday of the month worth knowing?

Because it fixes the whole layout. A 30-day month starting on a Tuesday ends on a Wednesday, leaving two weekdays with a fifth occurrence, and that decides whether a four-week shift roster fits without spilling. It also tells you at a glance whether the month starts on a weekend, which shifts every payday and delivery window in it.

• ISO 8601: International Standard Date and Time Notation — University of Cambridge Computer Laboratory
• The Gregorian calendar — Claus Tøndering, Calendar FAQ