In the musical, during the opening number, a seven-year-old Shrek passes by Fiona as she's being taken to the tower. Years later they reunite, with Shrek rescuing her from the tower, and in the musical number, "I Think I Got You Beat," Shrek restates that he was sent away on his birthday, and Fiona responds that she was sent away on Christmas Eve. Of course, it could have taken Shrek several days from leaving his parents' house to seeing Fiona, but if you assume that they both left home on the same day, then we also know that Shrek's birthday is Christmas Eve!
The only characters in the series who have Disney-style in-universe musical numbers, as opposed to scenes set to popular real-life songs, are Fairy Godmother (Shrek 2) and Prince Charming (Shrek the Third), and both of them actually do a stage-setting for those numbers (Fairy Godmother with magic, Prince Charming the old-fashioned way). And of course it makes sense as both a Deconstruction of Disney musical numbers, showing the need for stage-setting, and a Take That!, considering that both characters are villains.
Then there's Robin Hood in 1, with his entourage of backup musicians/dancers and his attempt to be a Hero of Another Story resulting in Van Helsing Hate Crimes, and then failing at that.
Donkey and Dragon's status as Shrek and Fiona's Beta Couple is reinforced by Shrek and Donkey's views on parenthood. Shrek is anxious when he hears that Fiona's pregnant and implicitly worries that he'll be a bad father due to the way his father treated him. By contrast, Donkey is absolutely thrilled when he discovers that he and Dragon have become parents and becomes a loving, doting dad to all his little baby abominations.
