The story behind it
All of you are aware that Mahalaya paksham started from 28th sep this year.
Am just sharing the reason and story behind it
Karna the great warrior after getting killed in Mahabharata war was received with honors at Heaven by Yama. He said you can enjoy the Heaven as you have done great penance in the birth.
Karna was happy and was enjoying the stint. After sometime he felt hungry and asked inmates where food is served. Heavenly abodes were taken by shock and informed that in Heaven you don't feel hungry and no food is needed for them. The deva guru Brihaspathi watching this went into deep meditation and asked Karna to lick his index finger. Karna did the same and hunger was gone. Karna was totally surprised and asked the reason.
Guru explained "Karna in your birth you were a "giver" and have done all charity to needed. But you were not inclined to Anna dhaana which is foremost and that is the reason you felt hungry in Heaven.
Karna asked "then how the same had gone when I licked my index finger?"
Guru explained "once a poor Brahmin came and asked food at your house. Since you generally don't do Anna Dhaana you refused but pointed out through your index finger a place where Anna Dhaanam was done. The Brahmin went there and had his food and got his hunger quenched. That 'Punya" was lying in your finger and hence you could feel the hunger had gone from you"!
Karna was in tears. He ran to Yama and pleaded for mortal form for one Paksha (fortnight). Yama asked him the reason and Karna said he wanted to do only Anna Dhaana in this Paksha. Yama was moved and granted him the same.
Karna came to Earth and did Anna Dhaana in a place where nobody could identify him. He was so happy that this time was spent for a great cause.
After a fortnight Yama came back and asked Karna to renounce the mortal body. Karna happily agreed.
Yama felt moved by his gesture and said "anybody would have asked more time to stay in Earth and enjoy pleasures here. But you kept up your words and did exactly for what your mortal body was granted. Please ask a boon which will be granted"
Karna said "Lord many people in Earth would have forgotten to feed their ancestors or anyone. Therefore if in this Paksha anyone offering food should reach the ancestors or souls who were deprived of offspring to do Karma for them"
Yama happily granted the same and took Karna to Heaven.
Yama said "anybody offering food in this Paksha is blessed and will also reach those who were not fortunate to be fed in Earth and Heaven"
Therefore please offer food in this Paksha the blessings of which will hold good for 21 generations
(Applicable for those who are having rights to do Pitru Karma)