At the traditional solar transition commemorated by Songkran, the sun enters Aries.
The name Songkran comes from a Sanskrit term meaning an astrological transition or movement. In the traditional calendar, the festival marks the sun’s passage from Pisces into Aries, beginning a new solar year.
This astronomical and calendrical idea helps explain why Songkran is celebrated in April rather than on January 1, the modern civil New Year in Thailand. The exact astronomical transition and the public holiday schedule are related but not identical concepts.
A common mix-up is treating Aries as a reference to a lunar phase. Songkran’s traditional timing is solar and zodiacal, while many other Thai religious dates are calculated using lunar-calendar rules.