A leap cycle of 28 years contains 7 leap days or 5 leap weeks and thus overall 10227 days and 1461 weeks or 336 twelfth-of-a-year months. Its mean year length is 365.25 days or 365 days and 6 hours. It is 7 Olympiads long, and it is the cycle after which the Julian calendar repeats its days of the week count.

## Leap rule

Usually, every fourth year has a leap day in it and these are the years whose year number is evenly divisible by 4, i.e. the the remainder is 0. The distribution of leap weeks is less systematic. If ISO 8601 rules are applied, the 53rd week is in years with the dominical letter D, DC or ED. The sequence is as follows:

1. G – cycle starts here if year count starts at 1 and 1 January shall be a Monday
2. F
3. E
4. DC: leap day and leap week
5. B
6. A
7. G
8. FE: leap day
9. D: leap week
10. C
11. B
12. AG: leap day – cycle starts here if year count starts at 1 and 1 January shall be a Sunday
13. F
14. E
15. D: leap week
16. CB: leap day
17. A
18. G
19. F
20. ED: leap day and leap week
21. C
22. B
23. A – cycle starts here if year count starts at 1 and 1 January shall be a Sunday
24. GF: leap day – cycle starts here if year count starts at 0 and 1 January shall be a Monday
25. E
26. D: leap week
27. C
28. BA: leap day