Adding a bit more to Trivia.
Generally Moon is at its 15th degree on Purnima. Each kala corresponds to 15 tithis or lunar days, starting from 1 on Pratipada or first lunar day of the lunar month to Purnima or Full Moon the 15th lunar day of that lunar month. On Amavasya or no moon day the moon is on 0 kala. However in this particular month Ashwin, the pitras get 1 degree or kala on Amavasya also and that's why the Purnima is of 16th degree despite being the 15th lunar day. That's why we have pitra paksha or sharad paksha in this lunar month