How would you inform guests that it's a 1920's theme wedding and let them know, if they would like, to join in on the fun and dress the part without them thinking it's required?
A cleverly worded invitation is the key to a successful theme wedding. First of all, set the stage for the entire event. For a 1920's soiree, you might choose an art deco-style invitation and font.
Then, rather than just saying "reception to follow," you could say "After the ceremony, please join us for a jazz-age inspired celebration. Dance the Charleston, and enjoy some speak-easy refreshments." Then, at the bottom, write either "1920s costumes encouraged" or "Dress: 1920s-inspired clothing or cocktail attire." For an extra added touch, consider having an assortment of fedoras, cloche hats, feather boas, and long beaded necklaces for those guests who didn't dress up, but wish that they had.