Day vs. Night
You might hear burners say they are day burners or night burners. This does not mean they are or aren't vampires, but implies their preference for the day time or night time atmosphere in Black Rock City. The two are so different it is almost like being in a different place. The following are my opinions and your mileage may vary - everyone burns differently.
A great time to do camp chores, get ice, socialize, view art, tour the side block camps (many of the small experiences at BM are more memorable than the big ones), or just cat nap in your shade structure. I find the day time vibe much more laid back.
As the temperature drops, things start to heat up. Noise camps start pumping it out, everything lights up, sobriety levels are in decline, and everything seems far more intense. Night brings on a lot more dancing, raves, fire, and general insanity.
Which To Choose
Either or both. Personally I like both equally, and typically party till 4 AM or so then sleep for a few hours and start my day. But then, I don't sleep much at the burn.