Many people are in full-blown Con-mode by Thursday. There are alot of pre-parties where people cosplay Thursday night, particularly for track-specific meet-and-greets, and I've seen some eager folks just wearing their cosplays Thursday afternoon while people are checking in and getting pumped (less common though). Friday morning onward, there is serious cosplay anywhere you look. Saturday is when the most people cosplay, or bring out the big guns, so to say, but plenty of people are dressed up the entire weekend. I've never actually stayed on Monday (unless you count walking to MARTA at 9am after staying up all night) because I usually work, so I can't say what Monday's like.
It's always Labor Day weekend, so yeah, the (Thursday!) Friday-Monday schedule is the norm.
Costumes for DragonCon 2013:
Steampunk Suffragette (original)
Spice (Batman Forever)