1951 — common year of 365 days, 1 January on a Monday

The rule for leap years

Divisible by 4 makes a leap year, divisible by 100 takes it back, divisible by 400 grants it again. That is why 1900 was not a leap year and 2000 was.

Days in each month

Years of 52 weeks and of 53

ISO counts a week as belonging to the year that holds its Thursday. A year runs to 53 weeks only when it contains 53 Thursdays — that is, when 1 January is a Thursday, or a Wednesday in a leap year.

The sixty-year cycle

Ten heavenly stems and twelve earthly branches advance together, one step each year, so the same pair returns only after sixty years. The animal alone repeats every twelve.

Nearby years

Decade by decade

How to read this

• Divisible by 4 is a leap year, by 100 is not, by 400 is again.
• A leap year holds 366 days, and February runs to the 29th.
• The ISO week count follows the Thursdays: 53 of them make a 53-week year.
• The sexagenary sign returns every 60 years, the animal every 12.