Govardhan Puja, also called Annakut, is the day after Diwali. It falls on Kartika Shukla Pratipada, the first day of the bright half of the month of Kartika, and recalls the story of Krishna lifting the Govardhan hill to shelter the people of Vrindavan. Temples and homes prepare a hill of food, often many dozens of vegetarian dishes, offer it to the deity and share it as prasad. In Gujarat and Rajasthan the day is also the start of the new year.
The date is fixed by the day on which Pratipada is in force in the Aparahna, the afternoon, following the published panchang we compared with; when it covers the afternoon of two days, the second is taken. The table lists the morning puja window, which starts at sunrise and runs for a few hours. Some families do the puja in the evening instead, and a few calendars use a different rule, so check with your family or temple.
Why do cities differ? Pratipada starts at the same instant worldwide, which for this festival is often just after the new Moon that ends Diwali. The afternoon comes at a different clock time in each city, and a tithi that has started before the afternoon in one city may start just after it in another. So some cities keep Govardhan Puja on the day after Diwali and some, in some years, two days after.
A calm note: sharing food is the point of the day, so it can be as small as a plate of sweets offered and passed round to neighbours. If your family or temple keeps a different day, follow them.