A Saga is a small subplot that takes place during any given time in Domus Facina, except during other Sagas. A saga can be as short as just three episodes to as long as twenty. The episodes that make up a saga usually take place directly in a row, but some episodes must be postponed for introductions, though it's entirely possible to work an introduction into a saga.
The first saga in Domus Facina was Sephiroth's Milk Saga, which lasted five episodes. The author has stated that each member of the main cast will eventually get their own saga, but that doesn't mean other characters can't have their own saga as well, short as they may end up being.
List of Sagas thus far: