My trip is in June and July. The beginning of my current Itinerary is:
Friday: flights to Vilnius. 1st night on the plane
Saturday: Arrive in Vilnius, Lithuania at 12:40pm. 2nd night. 1st night in Vilnius
Sunday: 3rd night, 2nd night in Vilnius
Monday: 4th night, 3rd night in Vilnius
Tuesday: Train or bus to Trakai, Lithuania. 5th night, one night in Trakai.
Wednesday 7/1: Trakai Island Castle (open 10-7), Karaite ethnographic museum (10-6?). Pick luggage up from the hotel. Bus or train to Kaunas, Lithuania. 6th night, 1st night in Kaunas.

Do you think I am making a mistake by spending a night in Trakai instead of just doing a day trip from Vilnius and spending an extra night in Vilnius?

I was thinking that if I arrive in Trakai in the evening and spend the night in a hotel in the town, I could wake up at 7:30 to 8pm and still get to the castle when it opens without having to rush around while if I do a day trip from Vilnius, I would have to wake up rather early, early enough to get to the bus station or train station, wait for the bus, and then the 30-40 minute trip, then squeeze in some breakfast somewhere, before I get to the castle. But so far I didn't see any bus trips from Trakai to Kaunas that don't involve backtracking to Vilnius to change buses and I didn't see a train trip from Trakai to Kaunas on the LTG website.