I happened to be working in the area of Uncle Tom's Cabin today so stopped around at the museum on my lunch break. It's not officially open for the season yet (opens on the Canadian Victoria Day long weekend in May) but I was pretty sure Steve and Brenda would be there, getting things ready and they were.
The website isn't updated with this information yet but I found out that their Emancipation Day celebration is July 31st this year. Just so you're aware...that is a long weekend in Ontario with the civic holiday of August 1st.
The Buxton National Historic Site has a Homecoming Weekend celebration but that is held on the Labour Day weekend in Sept.
Have you decided on a date yet for the rest of your trip?