I've gone the last two February dapper days. In 2014, the crowd was INSANE, worse than I have ever seen it at DL (DCA wasn't bad). It was pretty warm out that day and it was a weekend, so maybe that's why it was so high, but it was definitely busy. I should add the days around it were relatively empty, just that day was crazy. This last February, it wasn't bad at all, not even close to the crowds I saw in Feb 14'. For 2015, the weather sucked for dapper day, it rained most of the morning. The days around it were not that busy, totally manageable. I think the weather caused a lot of locals to skip attending.
Next month will be the first fall dapper day I've attended. It's on a Friday, so I am expecting crowds, but I assume it will start out manageable and just get busier as the day goes on and locals get off work, etc.