As the expression implies, a very warm/hot place. This effectively communicates that it's uncomfortably hot, just as a whorehouse would be from the droves of people who would show up for "dollar night."
Sam: Why is it so warm in here. Dinner will cost us $100 each; you'd think they can afford air conditioning!
Nora: Yeah, it's hot as a whore house on dollar night.
Nora: Yeah, it's hot as a whore house on dollar night.
by ChiDiva December 18, 2007