The full moon is the one everybody knows: culmination, high emotion, and the traditional night for gratitude, or for a good cry.

Every culture with a calendar has put its big nights here, because it's the one phase you can't miss and the only one that lights the ground. Festivals, harvests, and a great many rituals land on it for that reason alone. Modern practice reads it as things coming to a head, which is also what happens when everyone stays out late because it's bright.