Here's why:

* Lunar Cycle: The lunar cycle, or the time it takes for the moon to go through all its phases, is about 29.5 days.

* Monthly Calendar: Our calendar months are based on the Earth's rotation around the sun, which is roughly 30 days.

* Overlap: Because the lunar cycle and our calendar months are similar in length, there will always be at least one full moon within each calendar month.

It's possible that a full moon might occur very near the end of a month and then the next full moon could be very early in the following month, but there will never be a month entirely without a full moon.