Significance
Govardhan Puja, also called Annakut, commemorates Lord Krishna lifting the Govardhan hill on his little finger to shelter the people of Vrindavan from Indra's torrential rains, teaching devotion to nature over blind ritual.
It falls the day after Diwali and is the fourth day of the festival.
Rituals
Devotees build a small hill of cow-dung or food representing Govardhan, worship it, and offer an Annakut — a "mountain" of many cooked dishes — to Krishna. Cows are honoured and decorated, and in temples grand food offerings are made.
How the date is fixed
Govardhan Puja falls on the Pratipada of Kartika Shukla paksha, the day after Diwali (Amavasya).