Hi, Mark!
There is so much to enjoy at Walt Disney World, and sometimes times overlap so guests need to choose between experiences. It sounds like this is happening during your visit to Disney's Hollywood Studios.
Fantasmic! is an epic nighttime spectacular, with seating for thousands of guests, so even with reserved seating with the
Fantasmic! Dining Package you will want to be in the amphitheater by about 7:45pm if the show is at 8:30pm. Unfortunately, this means you are not going to be able to watch
Jingle Bell, Jingle BAM if at 8:00pm.
I wish I could tell you some magical, brilliant way to see both, but if you are only visiting Disney's Hollywood Studios one night, it just is not feasible. If time/budget allows, you could add the Park Hopper option onto your theme park tickets and visit a second night. Also, check the park schedules to see if there is a different night when you could enjoy both events.
If you miss Jingle Bell, Jingle BAM, make sure to plan fun holiday events for other nights such as
Mickey's Very Merry Christmas Party or
Candlelight Processional. You can definitely find
Holiday Fun all around Walt Disney World.
Finally, please remember that you simply cannot do everything in just one Walt Disney World vacation. Keep notes of all the fun things you missed that you wanted to do - a perfect reason to plan a return trip. See you real soon!
Cathy