Literally and schoolboyishly, how about: "Laissez rouler les bons temps"? I have no doubt the actual French have a totally different and idiomatic way of saying the same thing, though!
I tried 'Laissez rouler les bon temps' on a French Canadian friend of mine years ago and was told that it's not the correct phrase and is something that anglophones often use. I never did find out the correct wording :(
"Laissez les bon temps rouler" is the phrase frequently used in New Orleans.
I can't swear that the French or French Canadians would use the exact same expression to convey that notion, but francophone residents of Louisiana have made the phrase the unofficial motto of the Big Easy.
que la fette commence,literalt'that the party begin', meaning let the party begin. French'English Eng.fr. Oxford dictionary. sounds like spanish, que la fiesta comience.