1) The term Blue Moon can refer to the second full moon in a given month. When people say "once in a blue moon" they are speaking to the rarity of the occurrence of a second moon happening in the same given month.
2) The full moon occurs once every 29.5 days. This essentially means that there is one full moon every month. However, as we know, every month but February has at least 30 days in it, which presents the potential for two full moons in a month.
ENJOY & DON'T LET THE "CRAZIES" BOTHER YOU!! ;-)