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 wewe see them grow closer and drift apart. Katniss alisema in Mockingjay that she fell in upendo with Peeta the siku he through the mkate to her, but I don't think she truely relized she was in upendo 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 upendo with Peeta at an unknown time. But, Katniss realised she was in upendo while she was in the bunker in district 13 talking to Finnick at night.
I agree with reapingcoin, Katniss fell in upendo with Peeta when he threw the mkate 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.
