15 days of the Hindu month is considered as Pitra Paksha, which are days dedicated to ancestors. It occurs in the Hindu month of Bhadrapada i.e. September-October in the Gregorian calendar. This year (2017) Pitra Paksha starts on 10th September and ends on 19th September.
Hindus believe that our lives are controlled by Navagraha / 9 planets and by ancestors. Hence, during the Pitra Paksha the ancestors are remembered and worshiped.
During this period special offerings are offered to the departed souls like Cooked Rice, Black Sesame Seeds and Water known as Tarpanam / Tarpana is offered to the ancestors.
Each day in the fortnight is ruled by a particular Thithi / Moon Phase and each day the benefits of offering Tarpan are different.
Mahalaya Amavasya is the last day of the Pitra Paksha. On this day people donate food and clothes.
Auspicious occasions are not celebrated during this fortnight as it is reserved for the reverence of the departed.