I remember going to a swap in Nashville a year or so ago in January with snow on the ground. Everything went pretty well then.
From my observations over the years, there are several things to think about went attempting to put on a frag swap.
First, why would you want to do it? Do you intend to make money or do you just want to do it for kicks and giggles?
Have you considered how many tasks will be involved: securing a site, acquiring tables, contacting potential sponsors, publicizing the event, shopping for food and refreshments, etc.
How many people can you COUNT ON to be available to help setup before and cleanup after the event, to work the table at the door, to sell refreshments, etc.? If 4 people say they will help 3 months prior to the event you can probably count on 1 to show up.
Most people want to setup the night before the event to give their corals a chance to open up before the doors open.
I've seen several swaps that were done right and were successful. I've also been to a couple that were disappointing. I think it all comes down to having leadership and foresight in getting things done in a timely manner and just "being ready".
Good luck.