The traditional South Asian day is divided into eight prahars (also pahar), each roughly three hours: four for daytime and four for night. This eight-fold clock is the scaffolding of raag samay — every raag is associated with one prahar in which it is ideally heard.
The first prahar of the morning belongs to dawn raags; the first prahar of night, around dusk, to raags like Yaman; the deep-night prahars to Darbari and Maalkons. Knowing a raag's prahar tells a listener a great deal about its expected mood.
The same word survives in everyday South Asian languages for a stretch of the day, and in music it remains the natural unit for talking about when a raag lives.