A few tips from Woman's Day Magazine (Feb. 2010 issue, pages 24-28) on the best times to do things:
Buying clothes online: Wednesdays & Thursdays (steepest discounts)
Go to a theme park: September, least crowded, most deals available.
Go to the mall: End of a selling season (clearance!)
Go to the supermarket: Wednesday (least crowded)
Las Vegas: Late July - Early August (off-season = more deals)
To the bathroom during a movie: Check out Runpee.com for best times depending on the movie
Always sold-out Movie: A mon thru wed matinee
A mammogram: Right after your period/before you've had your morning coffee
Spay/Neuter a pet: Ages 2-6 months (to avoid urine marking/pregnancy)
A walk: After dinner (helps digest your food, relieves stress before bed)
A workout: 4pm-6pm
They elaborate in the magazine on exactly why these are the best times to do things so if you want to know more, pick up a copy!