MOONWHILE in Mississauga this evening I had the once in a blue moon chance to photograph the very rare Blue Moon. The phrase 'once in a blue moon' is derived from this type of a celestial event - i.e. 2 full moons in a single month - specialty here today being granted the extra spookiness on a Halloween night.. This October had the very rare occurrence of 2 Full Moons in a month - the last one being on 1 October 2020.