Answer: the birthday of their children
By this point in time, Shrek is starting to feel overly domestic and very much un-ogre like. His home is on a coach tour where the tour guide talks about how people used to be afraid of him. People ask him to autograph their pitchforks, which they once used to try and chase him away.
At the party an annoying child keeps pushing Shrek to "do the roar" and when Shrek finally does, people laugh and applaud rather than show fear. The birthday party has a cake (which the three pigs eat) that has a cartoonish picture of an ogre on it, which also offends Shrek. After the pigs eat the cake, Puss in Boots finds a second one. This angers Shrek and he smashes it. Fiona tries to talk to him but isn't terribly sympathetic about his attitude on their children's birthday.
Shrek is voiced by Mike Myers and Fiona by Cameron Diaz.