From Hindupedia, the Hindu Encyclopedia

A unit of time or season. Ritu equals two months where each month is composed of 30 days and each day of 60 muhoortams. A year has six ritus.

In the general sense ritu is used to refer to a duration of cyclic phenomenon - ritu refers to a season. Ritu carya is seasonal routine.

Menstrual cycle of woman is also called ritu - ritu kala is the menstrual period and ritu snana marks the end of it.