April 13 is Sidereal New Year or Mesha Sankranti, and it is observed in India and other countries that have been influenced by the Hindu calendar. The Sidereal Year is based on the position of the Sun in relation to the stars, and the New Year is when the Sun enters the constellation of Aries.
The Hindu observance includes washing in one of the holy rivers. However the day is also celebrated by Theravada Buddhists as the Lunar New Year and also involves water. It is called the Water Sprinkling Festival.