The Texas Butterfly Festival is based at Bentsen-Rio Grande Valley State Park in Mission, headquarters of the World Birding Center, and it runs in the fall at the peak of migration.
The Valley sits on a corridor that concentrates species you cannot reliably see anywhere else in the United States, and the festival exists because of that geography rather than in spite of it. It is built around guided field trips to sites across the four counties, led by people who know exactly where to stand and when.
If you have never been
This is a festival for going somewhere, not for standing in one place. The field trips are the event. Wear closed shoes, bring water and a hat, and expect to be out in the sun for stretches.
You do not need to know anything about butterflies to enjoy it, and the guides are used to that.