How the date is fixed
Govardhan Puja falls on the first day of the bright half of the lunar month Kartika (Shukla Pratipada). The date is the day on which that tithi is running at sunrise (the usual rule, called udaya tithi).
About
Govardhan Puja falls on Kartika Shukla Pratipada, the first tithi after the Diwali new moon, in October or November. The tithi at sunrise is taken; when Pratipada is a kshaya tithi the day is merged with Amavasya or Dwitiya by the almanac's rule, and Govardhan Puja then shifts by a day, which is the usual reason for the occasional gap between Diwali and Govardhan. Bhai Dooj follows on the next day, or on the same day when Dwitiya already covers the afternoon: Govardhan Puja takes the tithi of the morning, Bhai Dooj that of the afternoon. In Braj the day recalls Krishna lifting the hill; in Gujarat it is the new year of the Vikram samvat; in Maharashtra it is Bali Pratipada. Temples build the annakut, a mound of cooked food, before the deity.
How it is observed
A Govardhan mound of cow dung or food is made and circled, cattle are decorated, and annakut is offered in temples.