While Buffy is playing charades with the crowd at Xander and Anya's wedding, she puts her fingers up to her head like horns and you can hear someone in the background guess "Deathwok Clan!" This is a reference to Lorne from Angel (1999), who is a demon with horns and a member of the Deathwok clan.
Amber Benson said in an interview to Buffy magazine that "I'm the biggest klutz on the set. Remember Emma [Caulfield] in that wedding dress? I got that dress off of her at least four times, because I'd be following her and I'd step on the dress and she'd be pulling it back up. But she looked gorgeous in that dress."
This is the first and last time Xander's parents (and extended family) appear. Though, his mom has been heard on the phone and shouting downstairs, previously.
When D'Hoffryn enters the wedding and greets Dawn, he tells her "Hymen's greetings". Hymen was the Greek god of marriage ceremonies, inspiring feasts and songs.
After Willow and Xander see each other in formal wear, she jokes that it's a good thing she realized she was gay. This refers to Homecoming (1998), where she and Xander couldn't resist each other in formal wear and shared an illicit kiss.