Majjhima Nikaya 3:18
SUJATOTena kho pana samayena bhagavā tadahuposathe pannarase komudiyā cātumāsiniyā puṇṇāya puṇṇamāya rattiyā bhikkhusaṅghaparivuto abbhokāse nisinno hoti.
Now, at that time it was the sabbath—the Komudī full moon on the fifteenth day of the fourth month—and the Buddha was sitting in the open surrounded by a Saṅgha of monks.