There are six ritus (also transliterated rutu) or Indian seasons in the Hindu calendar. The following table gives an overview about the partititon of the year in the traditional Indian calendar.
It is not spelled Ruthu but unless you're Indian, you probably don't know how to pronounce the sound. It is sort of like the th in smith. ^-^ I know because my name is Ritu and I happened to pass by this page, so don't you dare say I'm wrong!!