There aren't that many good (or even popular) songs about Friday... Saturday, Sunday, and Monday seem to get much more pub. But AMG and AllOfMp3 were helpful in jogging my memory. I just picked out some by artsts I'm familiar with:

Ice Cube - Friday
Raekwon - Friday
Phish - Friday
Joe Jackson - Friday
The Easybeats - Friday on My Mind
G. Love and Special Sauce - Friday Night
Roy Orbison - Friday Night
Mister Cheeks - Friday Night
Fun Lovin' Criminals - Friday Night
Dubstar - Friday Night
Steely Dan - Black Friday
Black Crowes - Good Friday
Genesis - Get 'em Out By Friday
Megadeth - Good Mourning - Black Friday
The Turtles - Somewhere Friday Night

My apologies if some of these are covers of others, I have only personally heard about 2/3 of the above list. Also, some of the above aren't exactly wedding-friendly (can't remember the last time I heard a Megadeth request at a wedding.)