Purnima in December 2028

In Delhi: the fast on Saturday, 2 December, with the Purnima from Fri 1 Dec, 06:41 to Sat 2 Dec, 07:10; the fast on Sunday, 31 December, with the Purnima from Sat 30 Dec, 23:14 to Sun 31 Dec, 22:18.

The full moon: the fifteenth tithi, which ends the bright half (Shukla) of the lunar month. A tithi is the time the Moon takes to gain 12° on the Sun. In the pūrṇimānta reckoning the Pūrṇimā is the last day of the month. Surya Siddhanta 2.64, 2.66

The fast is kept on the later of the two days, the one joined to the next Pratipadā, and never on the day joined to Chaturdaśī. The Sāvitrī vrata is the exception and keeps the earlier day. Nirṇaya-sindhu p. 73

Some families keep the Pūrṇimā on the earlier day, joined to Caturdaśī, as a family custom (kula-dharma). The Dharmasindhu says the basis for doing so every month is still to be sought, and accepts the earlier day only when the Caturdaśī lasts under eighteen nāḍīs (7 hours 12 minutes) after that day’s sunrise. These pages show the earlier day only then, labelled family custom; the fast itself is the later day, by the rule of the Nirṇaya-sindhu. Dharmasindhu p. 20

Each date is worked out for the city’s own sunrise. The Pūrṇimā begins and ends at the same moment everywhere, but which sunrise it reaches depends on the city, so cities far apart can keep different days.