What is "thrayodashi"

Thrayodashi is the Sanskrit word for "thirteen", and is the thirteenth day in the lunar fortnight ( Paksha) of the Hindu calendar. Each month has two Thrayodashi days, being the thirteenth day of the "bright" (Shukla) and of the "dark" (Krishna) fortnights respectively. Thus Thrayodashi occurs on the thirteenth and the twenty-eighth day of each month.