Hoepfully not going to derail this:
ALWAYS double check for big events when booking shows. Your milage and whatnot may vary depending on where you live.
* Big holiday weekends in the States are death for gigs (usually). Labor Day, Memorial Day, 4th of July, Thanksgiving, and Christmas are all awful weekends to gig. More often times than not people are out doing other things. You'll wind up playing to the two locals that practically live at the bar and a few of your most rabid fans. Been there, done that. Don't want to do it again.
* Major sporting events. Like above Super Bowel story. Also be wary of March Madness weekends (especially early on when there are games all friggin' day long). People drink all day watching basketball and then go home. All other major professional sporting events should be considered.
* Local sports near your gig. In Seattle all we have to worry about anymore is football (proper and otherwise) and baseball. Both are located near music venues and that can really make parking a hassle, load in a bigger hassle, and just plain getting there a royal pain in the ass. Though this can also work in your favor after the sporting event is over. Thousands of drunken fans hitting the bars and packing the place.
Also - as noted about about pay and going after a bar for breach of contract: so not worth it. When you start getting into larger sums of money that's when a contract will be more important. Like those New Year's Eve gigs, corporate parties, weddings, casinos that have a budget, etc...