Each raga has a specific time at which it is performed. This is because those notes are supposed to be more effective at that particular time. The 24 hours of the day can be divided into two parts:
From 12 AM to 12 PM: this is called the Poorva Bhaag and the ragas sung in this period are called poorva raga.
From 12 PM to 12 AM: this is called the Uttar Bhaag and the ragas sung in this time frame are called Uttar raga.
Furthermore, the saptak also changes according to the period of the day. For example, in the Poorvang period, the saptak is from Sa to Ma (Sa, re, ga, ma). On the contrary, in the Uttarang period, the saptak is from Pa to Sa (pa, dha, ni, sa).