A Blue Moon is the second full moon in a calendar month. Usually months have only one full moon, but occasionally a second one sneaks in. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month. A blue moon occurs about once in 2.5 years on average.

A blue moon can occur in January and the following March if there is no full moon at all in February, as is the case in the years 1999, 2018, and 2037.

Note that because the full moon occurs at different times (and therefore potentially on different calendar days and in different calendar months) in different time zones, the occurrence of blue moons is time zone (and daylight saving time) dependent

1. 2001, Nov 1, Nov 30
2. 2004, Jul 2, Jul 31
3. 2007, Jun 1, Jun 30
4. 2009, Dec 2, Dec 31
5. 2012, Aug 2, Aug 31
6. 2015, Jul 2, Jul 31
7. 2018, Jan 2, Jan 31 (!)
8. 2018, Mar 2, Mar 31 (!)
9. 2020, Oct 1, Oct 31
10. 2023, Aug 1, Aug 31
11. 2026, May 1, May 31
12. 2028, Dec 2, Dec 31
13. 2031, Sep 1, Sep 30
14. 2034, Jul 1, Jul 31
15. 2037, Jan 2, Jan 31 (!)
16. 2037, Mar 2, Mar 31 (!)
17. 2039, Oct 2, Oct 31
18. 2042, Aug 1, Aug 31
19. 2045, May 1, May 30
20. 2048, Jan 1, Jan 31
21. 2050, Sep 1, Sep 30
22. 2053, Jul 1, Jul 30
23. 2056, Mar 2, Mar 31
24. 2058, Oct 2, Oct 31
25. 2061, Aug 1, Aug 30
26. 2064, May 1, May 30
27. 2066, Dec 1, Dec 31
28. 2069, Sep 1, Sep 30
29. 2072, May 2, May 31
30. 2075, Apr 1, Apr 30
31. 2077, Oct 2, Oct 31
32. 2080, Jul 2, Jul 31
33. 2083, May 2, May 31
34. 2085, Dec 1, Dec 31
35. 2088, Sep 1, Sep 30
36. 2091, Jul 1, Jul 30
37. 2094, Jan 1, Jan 31
38. 2096, Oct 2, Oct 31
39. 2099, Aug 1, Aug 30
40. 2102, Jun 1, Jun 30
41. 2104, Dec 2, Dec 31

     

The mean time intervall is 2.577 years (2 years 7 months).

Gregorian year = 365.2425 days

lunation: 29.53058867 days

12,368277 lunations per year

0.368277 extra lunations per year

1 / 0.36827 years = 2.7153 years per extra lunations (2 years 8.6 months).