I'd really recommend talking to the organizers. And possibly watching one of their normal events.
Here's some of my thoughts on potential biggies:
-You and the band will likely need to supply and run your own sound system.
-Many people will likely keep the same partner throughout the evening.
-The expected flow of the evening is very different -- typically they're used to doing two dances with the same partner of about 6-10 (?) minutes, and then a rest break. Potentially involving snacks. Rinse and repeat.
-Swings are neither expected nor required. Without further instruction, they'll go just once around, twirl, and stop.
-Be very careful about too much consecutive clockwise motion.
-You may have to adjust for the age level of the group. In general, MWSD groups tend to be older.
-They're really not used to hearing the phrase of the music.
-Dropping out the calls may be traumatic for some.
-Contra is much more aerobic than squares.
Good luck,
-Chris Page
San Diego, CA