The traditional New Year in many South and Southeast Asian cultures is based on the sun's entry into the constellation Aries.
As most countries and cultures of South and Southeast Asia lie within the Indian cultural sphere, the development of their traditional calendars have been strongly influenced by some form of the Hindu calendar.
As in many other calendars, the New Year was based on the northern hemisphere vernal equinox (the beginning of spring).
Today, the sun's entry into Aries occurs around 18 April, according to astronomical definitions.
The sun's entry into Aries is known as meṣa saṅkrānti in Sanskrit, and is observed as Mesha Sankranti and Songkran in South and South-east Asian cultures.