Each lunar month in Hindu calendar has two Chaturthi Tithis. The one after Purnimasi or full moon during Krishna Paksha is known as Sankashti Chaturthi and the one after Amavasya or new moon during Shukla Paksha is known as Vinayaka Chaturthi.
Although Sankashti Chaturthi fasting is done every month but the most significant Sankashti Chaturthi falls in month of Magha according to Purnimant School and in month of Paush according to Amavasyant School.
If Sankashti Chaturthi falls on Tuesday it is called Angarki Chaturthi and it is considered highly auspicious. Sankashti Chaturthi fast is mostly observed in Western and Southern India especially in Maharashtra and Tamil Nadu.
Notes: All timings are represented in 24+ hour notation in local time of New Delhi, India with DST adjustment (if applicable).
Hours past midnight are higher than 24:00 and fall on next day. In Panchang day starts and ends with sunrise.
Devotees of Lord Ganesha keep fast from sunrise to moonrise on Sankashti Chaturthi. Sankashti means deliverance during troubled times. Lord Ganesha, the supreme lord of intelligence, symbolizes the remover of all obstacles. Hence it is believed that one can get rid of all obstacles by observing this fast.
The fast is supposed to be strict and only fruit, roots (the part of a plant which is below ground) and vegetable products are supposed to be consumed. The staple Indian diet on Sankashti Chaturthi includes Sabudana Khichadi, potato and peanuts. Devotees break the fast at night after sighting of the moon.
In North India Sankashti Chaturthi during Magha month is known as Sakat Chauth. Also Vinayaka Chaturthi during Bhadrapada month is known as Ganesha Chaturthi. Ganesha Chaturthi is celebrated by Hindus all over the world as the birthday of Lord Ganesha.
Sankashti Chaturthi is also known as Ganesh Sankatahara or Sankatahara Chaturthi among Tamil Hindus.
It is important to understand that fasting day for Sankashti Chaturthi might differ for two cities even if those cities are within same Indian state. Fasting for Sankashti Chaturthi depends on moonrise and it is observed on the day when there is moonrise while Chaturthi Tithi is prevailing. Hence Sankashti Chaturthi fasting might be observed on Tritiya Tithi i.e. one day before Chaturthi Tithi. As moonrise time is different for all cities it is important to refer Hindu calendar like this website which lists location based Sankashti Chaturthi days. As generating location based dates are time consuming most sources ignore this fact and publish single list for all Indian cities.