Every 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 major 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 careful highly positive. Sankashti Chaturthi fast is mostly observed in Western and Southern India especially in Maharashtra and Tamil Nadu.