"Join us, Matt and Allison are tying the knot! take the plunge with them Saturday, June 22nd, at their Engagement/Beach Party, there'll be no chance to get cold feet at this sizzling hot summer party! takes place at Turtle Beach, this part is not stuffy ceremony, so join in if you want to see the couple let their hair down one last time before the walk down the aisle \ RSVP by Friday , June 15th, It'll be a blast"
I checked in the dictionary: "let your hair down" means: to enjoy yourself and start to relax, especially after working very hard
"go/walk down the aisle" means: to get married
but, I don't get the exact meaning of the sentence in bold. in other words, I don't understand why they let their hair down one last time. could you please explain it?