The period between New Moon and Full Moon in Simha Maasa (approximately in September)for 15 days is named as Mahalaya Paksha.

Lunar Calendar of Hindus divides a month with reference to the waxing and waning of the Moon,into two.(for details on Hindu/Indian Calculation of Time’ please read my Blog’TIME-Non-Linear Theory,filed in Astrophysics).

The waxing period is called Sukla Paksha(Sukla in Sanskrit means White,  here indicates the waxing of the Moon),ending in Full Moon, Pounima, also called Pournamasya.

The waning period is called Krishna Paksha( Krishna means Black),culminating in New Moon called Amavasya.

Each Paksha has 14 Days ,one Amavasya and one Pournima makes a Month.

One is expected to perform the duties due to the Departed daily, that is do Tharppana daily.

This applies all Castes, Brahmana,Kshatriya ,Vaisya and Sudra.

We have conveniently omitted to follow the Sastras, by saying that the Srardha and Tharppana  are to be performed only by  Brahmins .

We also have made it a habit of performing Tharppana only on Amavasya,Beginning of the New Month,Mahalaya ,Aadi ,Grahana( Eclipses)  periods and Srardha Thithi.

This Behavior is not  sanctioned by The Sastras.

Of all The Tharppanas ,Mahalaya Tharppana is very Special.

This period falls in The Dakshinayana-movement of the Sun to Southern Latitudes ,this is for six months.

This begins from Adi and extends up to Makara Sankaranthi ( January 14)

https://ramanisblog.in/2011/09/13/mahalaya-amavasyatharpana-explained/