A full moon doesn't happen on a specific day of the month. It's a phase of the moon that occurs roughly every 29.5 days, so it can fall on any day of the month.

Here's why:

* Lunar Cycle: The moon's cycle (from new moon to full moon and back again) is about 29.5 days long.

* Calendar Months: Our calendar months are not perfectly aligned with the lunar cycle. Some months have 30 days, some have 31, and February has 28 or 29.

Therefore, the full moon can occur anywhere from the 1st to the 31st of a month, depending on when the last new moon occurred.