It's not much of a song, but Jack's too giddy-cheerful not to sing-song it as he's finishing up the last touches on the pre-ceremony dinner. The Luggage can help with reception food, sure, but this is different. At Ianto's suggestion and since they're going for more formal than other dinners, they're already dressed up for it. A nice but light three course dinner, then the ceremony itself-- "WEEEDDIIING!"-- to conclude at sunset, and the reception under the full moon. Jack sing-songs in his tuxedo over to make sure the cake is ready to go for the reception as well, then figures it's all ready.
"Now we just need our guests and we'll eat!" He grabs up Ianto for a twirl around the kitchen while they wait. "Wedding!"