Pitru Paksham is an important duty to be done by every Hindu towards his ancestors (this does not apply to people whose parents are alive)

It falls during the Hindu month of Bhadrapada which is the next fortnight after Ganesh Chaturthi (Vinayak Chartuthi). It is for a period of 16 days and ends with the day of Mahalaya Amavasya (no moon on that day)

It is believed during this period the ancestors  decent to this material world  to bless and seek their share of  food from their own family members (the normal practice of Tharpanam that needs to be done every month may or may not have happened and this Mahalayam compensates for all that needs to have been performed)

The ceremony is performed with reciting of mantras and by distribution of food grains & with other essentials.

Performing this ceremony brings  prosperity, better health & peace of mind to all family members as the ancestors   are appeased and bless us

For people who find it difficult to perform annual ceremonies due to professional and financial constraints in their daily life can do this as a yearly activity covering the annual shradham too.

Who should perform?

Pitru Paksham needs to be  performed by the eldest son (does not forbid other sons to do)  for the Paternal side (father side) of the family for the  previous 7 generations (many cases only 3 generations are known), in case of  no sons in the family, the daughter has the right to perform (The procedure for same is  different)