How the date is fixed
Krishna Chaturthi, the fourth day of the dark half, kept on the day the tithi is running at moonrise; the fast is broken after sighting the Moon. One that falls on a Tuesday is called Angarki.
About
Sankashti Chaturthi falls on the fourth tithi of the dark half of every lunar month, giving twelve or thirteen a year. The governing moment is moonrise: the day on which Chaturthi is present when the Moon rises, usually between 8 and 10 pm, is chosen, and almanacs print the local moonrise. When it falls on a Tuesday it is called Angarki and is held in special regard in Maharashtra. The bright-half counterpart, Vinayaka Chaturthi, is observed at midday instead.
How it is observed
A fast through the day, Ganesha worship in the evening, and the fast broken after seeing the Moon.