This month is based upon the phases of the Moon. It is of two types: -
Poornimaant Maas: -
The time period from one Full Moon day (Poornima) to the next Full Moon Day is called as 'Poornimaant Maas' & is used in North India.
Amaant Maas: -
The time period from one Black Moon Day (Amaavasya) to the next Black Moon Day is called as 'Amaant Maas' & is used in South India.