What is the effect of celebrations?

[Dragon]

The manual says that if the people in a city are very happy, they may hold a celebration. Indeed they do; I get messages that say so. But I can't figure out what the effect is -- the manual doesn't say, and the word "celebration" is not in the game data files.

In Civ 2 the easy way to win is by getting "We love the President" days to grow population fast. Is there something similar in CTP II?