Devotees of Lord Ganesha fast on Chaturthi Tithi (fourth day) which falls on the Lunar fortnight. The fasting is done during the Shukla Paksha which is during the moon's waxing phase. This year Krishnapingala Sankashti Ganesh Chaturthi vrat will take place on June 17.
On this day, devotees pray to Krishna Pingala Maha Ganapati who is the avatar of Lord Ganpati and Sri Shakti Ganapati Peetha. Devotees observe a long fast, perform puja of Lord Ganpati post seeing the moon. According to legends, the importance of this vrat was told to Pandava King Yudhishthir by Shri Krishna in the Dwapar Yuga.
As per another mythology story, there was a king named Mahajit, who was kind and treated everyone like his family. One day a sage named Lomash heard his issue and told the king to observe Kirshnapingala Sankashti Chaturthi during Ashadha month. Lord Ganesha got happy and blessed him with a child. The king got happy and observed fast on every Sankashti Chaturthi.