Calendar

7 months have 31 days, 4 months have 30 days and February has 28 (29 in a leap year).

• E.g. 7 × 31 + 4 × 30 + 28 = 217 + 120 + 28 = 365.

A year that is divisible by 4 is a leap year (in 2000 to 2099).

• E.g. 2028 is divisible by 4, so it is a leap year. 2027 is not divisible, so it is a common year.

A week has 7 days. 365 days = 52 weeks and 1 day. 366 days = 52 weeks and 2 days.

• E.g. 52 × 7 = 364 and 365 − 364 = 1.

Knuckle trick : on a closed fist count from January as knuckle, valley, knuckle, valley. A knuckle month has 31 days, a valley month has 30 days (February is different). After July start again from the first knuckle for August.

• E.g. July and August are both 31 days one after the other.

Divide the number of days by 7. Move ahead as many weekdays as the remainder.

• E.g. Monday and 10 days later : 10 ÷ 7 = 1, remainder 3. Monday + 3 = Thursday.

If the days are a multiple of 7 (7, 14, 21, 28 …) the same weekday comes.

• E.g. Saturday and 28 days later : remainder 0, Saturday.

To go back, move back as many weekdays as the remainder.

• E.g. Wednesday and 12 days before : remainder 5. Five weekdays back from Wednesday : Friday.

When dividing a big number by 7, use a near multiple of 7. E.g. for 100 ÷ 7 subtract 98 (14 × 7).

• E.g. 100 − 98 = 2. Remainder 2.

In one month, dates that differ by 7 have the same weekday.

• E.g. If the 5th is a Tuesday, the 12th, 19th and 26th are also Tuesdays.

Find the difference between two dates of the same month, divide by 7 and move ahead as many weekdays as the remainder.

• E.g. The 5th is a Tuesday. The 30th : difference 25, remainder 4. Tuesday + 4 = Saturday.

For the same date next month add the days : a 31-day month moves the weekday 3 ahead, a 30-day month moves it 2 ahead, a 28-day month 0 and a 29-day month 1 ahead.

• E.g. If 10 January is a Sunday, then 10 February : January has 31 days, so 3 ahead : Wednesday.

To go two or more months ahead, keep adding the shift of each month (or count the total days and divide by 7).

• E.g. 20 January is a Friday; up to 20 March (common year) : 31 + 28 = 59 days, remainder 3 : Monday.

In a 28-day month every weekday comes 4 times.

• E.g. February of a common year has 4 Sundays.

Extra days = days of the month − 28. Starting from the weekday of the 1st, that many weekdays come 5 times.

• E.g. A 30-day month starting on Saturday : extra 2, so Saturday and Sunday come 5 times.

If the date of the first Sunday is known, keep adding 7, up to the days of the month.

• E.g. First Sunday on the 3rd : 3, 10, 17, 24, 31 (the fifth Sunday if the month has 31 days).

Think backwards : if you know which weekdays come 5 times, the 1st is the first of those weekdays.

• E.g. In a 31-day month Fri, Sat, Sun come 5 times : the 1st is a Friday.

Weekday of 1 January : after a common year it is 1 ahead, after a leap year 2 ahead.

• E.g. 2025 is a common year : if 1 January 2025 is a Wednesday, then 1 January 2026 is a Thursday.

To count days between two dates, add the remaining days of the current month, the days of the full months and the date of the last month.

• E.g. 10 March to 25 May : remaining days of March 21 + April 30 + 25 = 76 days.

Divide the total days by 7, see the remainder and move ahead that many weekdays.

• E.g. 76 ÷ 7 = 10, remainder 6.

After getting the answer, check in the opposite direction : subtract the remainder from the weekday of the answer and see if you reach the original weekday.

• E.g. Sunday − 6 = Monday.