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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
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