Ritu (Sanskrit: ऋतु) means "season" in different ancient Indian calendars used in India, Bangladesh, Nepal and Sri Lanka.
The word is derived from the Vedic Sanskrit word Ṛtú, a fixed or appointed time, especially the proper time for sacrifice (yajna) or ritual in Vedic religion; this in turn comes from the word Ṛta (ऋत), as used in Vedic Sanskrit literally means the "order or course of things".
Thai Pongal, Sankranti are celebrated during the month of Pausha.
East Indian calendars (Bengali, Assamese, Odia and Mithila) start their new year on Mesh Sankranti.
The season names corresponds to the Sanskrit Vasanta, Grishma, Varsha, Sharada, Hemanta, Shishira order.