In the book Ceasar Flickerman says he thinks that it was when the two of them were in the arena and Katniss yells for Peeta. Through out the book آپ see them grow closer and drift apart. Katniss کہا in Mockingjay that she fell in love with Peeta the دن he through the روٹی to her, but I don't think she truely relized she was in love with him until after her talk with Finnick when the capital bombs district 13, when he gives her the rope to tie knots with.
She fell in love with Peeta at an unknown time. But, Katniss realised she was in love while she was in the bunker in district 13 talking to Finnick at night.
I agree with reapingcoin, Katniss fell in love with Peeta when he threw the روٹی to her back when they were kids. I think she realized that she loved him when she was training for the Quarter Quell and was going to do everything in her power to let Peeta be the one to come out as a winner.
