3rd weekend in May every year
We went last year and had a blast! Mainstreet is in the historic district of St. Charles, on the river. The area (several blocks long) is breathtaking. The buidlings have been restored, the roads are brick, and there are great views of the river, a riverwalk, and flower beds everywhere.
Lewis and Clark Heritage Days is a historical re-enactment that will appeal to a variety of ages. There is a parade, a model of the boat, people in costumes, carriages, a historical church service on Sunday, themed vendors and food, historic games for kids to play, a weapons demonstration. The idea is to keep the weekend feeling like it's 1804 and Lewis and Clark have arrived.
The historic district is filled with restaurants, cafes, and shops. (As a side note, one of the greatest quilting shops I've every been to is there as well). There is a couple B&B's and outside the historic district there are many hotels to choose from. There are several historic points of interests along Main Street.
My son loved the parade of soldiers and weapons demonstration. If you are planning to be in the area the 3rd weekend of May it is totally worth it.
It is very close to St. Louis, so you can roll a trip to the Arch into this same trip easily.