Here's why:

* Lunar Cycle: The moon's cycle, from new moon to full moon and back again, takes approximately 29.5 days.

* Calendar Months: Most calendar months have either 30 or 31 days.

* Timing: This means that the timing of the full moon can sometimes shift into the following calendar month.

Therefore, you'll have months with two full moons (a "blue moon") and months with none.