Gangotpatti, also known as Ganga Pujan or Ganga Saptami, is observed on the seventh day (saptami) of the Shukla Paksha (waxing phase of moon) in Vaisakha month as per traditional calendar Hindu calendar. In 2015, the date of Ganga Pujan and Ganga Saptami is April 25. It is believed that
Ganga descended on to earth on the day and is mentioned in the Padma Purana and Narada Purana.
It must be noted here that some regions
Ganga is believed to have appeared on earth on Akshaya Tritiya day and some during Ganga Dusshera in Jyeshta month. Ganga Dusshera is a week-long celebration.
You can find more details about Ganga Saptami in this article.