Ritu (Sanskrit: ऋतु) means "season" in different ancient Indian calendars used in India, Bangladesh, Nepal and Sri Lanka. There are six ritus (also transliterated ritu) or seasons. Seasons are different times of the year and there are 12 months in the year. Every month has its own special season. 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". This word is used in nearly all Indian languages.

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Nepal and India observes six ecological seasons.^[1][2]

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. The Bengali Calendar is similar to the Sanskrit calendar above, but differs in start and end times which moves certain dates/days around (i.e., Vasant Panchami occurs here in Vasant ritu but in the calendar above, it occurs in Shishir as that is the Magha Shukla Panchami). The East Indian Calendar has the following seasons or ritus:

The seasons are described in literature such as the Sanskrit poem Ṛtusaṃhāra written by the legendary Sanskrit poet Kālidāsa.

Names of the ritu are commonly used for persons: typically, Vasant, Sharad, Hemant, Shishir and Varsh are "male" names; "female" names include Vasanti, Sharada, Hemanti, Grishma and Varsha.

Similar naming conventions are also used in Tamil: For female Ilavenil. For male Kar(Vannan).

• Astronomical basis of the Hindu Calendar
• Vedic timekeeping