How many full moons are in one year?

Depending on the phase of the moon on January 1st, there can be 12 or 13 full moons in any calendar year.

The average synodic month (period between New Moons) is 29.530589 days.

12 synodic months is 354.367068 days; about 11 days less than a year.

13-Month, 28-Day Calendar

There are 13 months in a year (13 x 28 = 364). And that at the end of the year there is a single extra day (364 + 1 = 365 - the same length as the average Gregorian year).

It was announced that on January 1, 1933, we would return to 13 months of 28 days each. But the Vatican resisted to it by focusing on a campaign against the "Null Day."

January - 11
February - 12
Ophiuchus/Orion 13
March - 1
April - 2
May - 3
June - 4
July - 5
August - 6
September - 7
October - 8
November - 9
December ‐ 10

..

SPRING
March - 1
April - 2
May - 3
June - 4

SUMMER
July - 5
August - 6
September - 7

FALL
October - 8
November - 9
December ‐ 10

WINTER
January - 11
February - 12
Ophiuchus/Orion 13
March - 1