In the Mahabharat ,
Karan ( son of the sun god) born with good armour which could not be pierced with anything, hence this made him immortal.
Karan was “ giving “ & generous person.
Lord Indra asked for his gold armour and without blink of eye he gave this special armour which left him defenceless.
In Mahabharat Karan fought on the side of Kaurav and died in the battle on the 17th day

In heaven Karan was treated very well as he was good person on earth. He recieved much more than what he donated while he was alive. But he realised every time Karan asked for food he got gold and gems, and this left him perplexed and he ask Lord of Death “ Yam” why do I not recieve food ?

To which Yam replied on earth you have distributed so much and here it’s multiplying in heaven, but you didn’t donate any food hence you recieve none here. Karan prayed to be sent to earth to undo this. He was sent to earth for few days.. he donated loads of food to the needy . He went back to heaven and Recieved loads food.

This visit of Karan to Earth was during the Pitru Paksh, hence during this period we learn from Karan and donate lots of food to needy and remember our ancestors.

Pitru means “ancestors “
Paksh means “ fortnight”
Hence during Pitru paksh we don’t celebrate or do anything suspicious but focus of donations and charities whilst remember our ancestors.