In north Indian classical music tradition it is specified that raags should be sung or played in particular times of the day, seasons, or holidays; this is called samay. It is said that appropriate performance may bring harmony, while playing or singing at different times may bring disharmony. It is said that the great Tansen was able to create rain by singing a monsoon raag.
According to swar and samay(time) Hindustani raagas are classified into three divisions.
1. Sandhi prakash raaga (raagas with komal “Re-Dha”)
2. Raagas with shudh “Re–Dha”
3. Raagas with komal “Ga-Ni”
1. Sandhi Prakash Raaga: In the above three points, the first point i.e, raagas with komal “Re-Dha” comes under sandhi prakash raagas, but we should keep in mind that in this division; with komal “Re-Dha” presence of shudh “Ga” is compulsory. If in place of shudh “Ga” there is komal “Ga” then it will come under third division. The time just before sunrise and time just before sunset is the time which is called sandhi prakash and raagas which are specified to be sung and played on this time are called sandhi prakash raagas. For example raag bhairav, raag bharavi, raag purvi, raag marwa. Sandhi prakash is also divided in two divisions.
a. Morning sandhi prakash raagas.
b. Evening sandhi prakash raagas.
Raagas which are specified to be sung and played during sunrise are called morning sandhi prakash raagas and raagas which are specified to be sung and played during sunset are called as evening sandhi prakash raagas.
2. Raagas With Shudh “Re-Dha”: Time for raagas with shudh “Re-Dha” comes after sandhi prakash raagas. Time for singing sandhi prakash raagas comes two times in 24 hours and raagas with shudh “Re-Dha” are sung or played after sandhi prakash raagas. So time for singing raagas with shudh “Re-Dha” also comes two times in 24 hours. In this division raagas from Kalyan, Bilawal, and Khamaj thaat are sung and played. Raagas with shudh “Re-Dha” are sung and played from 7am to 10 am and 7pm to 10 pm, in this division presence of shudh Ga is compulsory. In this division swar Ma is also very important. Raagas which are specified to be sung 7am to 10 am in those raagas there is presence of shudh Ma and raagas from 7pm to 10pm has presence of tivra Ma.
3. Raagas With Komal “Ga-Ni”: Time for singing and playing raagas with komal “Ga-Ni” comes after the time of raagas with shudh “Re-Dha” i.e. time for raagas komal “Ga-Ni” is from 10am to 4pm and 10pm to 4am. In this division identification of raagas is that, in these raagas komal Ga is compulsory weather Re and Dha are shudh or komal.