This episode was broadcast 4 months before the U.S. Supreme Court handed down a decision that would open the doors to same-sex marriage. Ever since, June 26 is now celebrated as "National Equality Day" or "National Marriage Equality Day".
When Kurt & Blaine are talking to Burt and Carol about their wedding, they say, "It was a beautiful day," a couple of times. That was the song the cast sang at that wedding.
Gina Gershon portrays Blaine's (and therefore, also Cooper's) mom. The age differences between the actors is 24 years between Gershon and Criss, but only 15 years between Gershon and Bomer.