I deeply apologize for not have very many sources, however I remember them stating in the first book about how the game makers would change the temperatures, (in the first book, they had to deal with crazy temperatures and humidity and then freezing cold, also they included the feast, to speed things up.
From the movie:
Attention tributes, attention! Commencing at sunrise, there will be a feast tomorrow at the Cornucopia. This will be no ordinary occasion. Each of you needs something. Desperately. And we plan to be... generous hosts.
From the book (The Hunger Games, chapter 18):
But occasionally, there will be trumpets followed by an announcement. Usually, this will be a call to a feast. When food is scarce, the Gamemakers will invite the players to a banquet, somewhere known to all like the Cornucopia, as an inducement to gather and fight.
Last among this is that if it takes to long, eventually the place will run out of food, making the tributes die of thirst or starvation.
