Sankashti Chaturthi is a monthly fast for Ganesha, kept on the fourth day (Chaturthi) of the dark half of each lunar month. Devotees fast through the day, pray to Ganesha in the evening and break the fast after the Moon is seen and offered water. It comes 12 or 13 times a year, and each month has its own name for Ganesha, such as Lambodara or Vikata.
The rule is based on the Moon: the vrat is kept on the civil day on which Chaturthi is in force at moonrise in that city. The moonrise time in the tables is worked out for each city with the Moon’s centre on the horizon, the same convention as a widely used panchang, which we checked it against. In a few months Chaturthi is over before that day’s moonrise and the vrat goes by the moonrise of the Hindu day, so a moonrise near midnight or after can appear.
Why do cities differ? Moonrise is at a different clock time in every city, often by an hour or more, and depends on latitude, which matters more at London’s latitude than at New Delhi’s. The tithi may cover the moonrise in one city and have ended in another, so the vrat date can differ, and the moonrise time always does. The moonrise in your own city is the useful number.
A calm note: fasting is a personal choice. If you have a health condition, are pregnant, nursing or take medicine, speak to your doctor about how to keep the vrat. The tables below cover five cities for both years. Every one of the 49 cities has its own page with all Sankashti Chaturthi dates and moonrise times.