Buddhist observance days fall on the new moon, the full moon and both quarters, a four-times-a-month rhythm kept for roughly 2,500 years.

Uposatha days are for renewing precepts and going to the monastery; for monastics they are when the rule is recited aloud. It is one of very few traditions anywhere that gives the quarter moons equal billing with the full one, and it means a great deal of Southeast Asia still runs part of its calendar off the phase rather than the date.