Stephen:
For several years, the Hallowishes Fireworks for
Mickey's Not So Scary Halloween Party have started at 9:30 p.m. When we went this past October, we found that same time to be accurate. Although I can't guarantee that Disney will have the same schedule this fall, I would think you'd be safe to go with that time.
I always recommend that folks allow at least 1.5 hours for a Signature Dining Experience. (However, we've had a fabulous dinner at
Narcoossee's last well over two hours.) If you want to simply see the fireworks from the dining room of
California Grill, then a seating time after 8:00 p.m. would allow for it. However, if you'd like to see the fireworks from the overlook platform outside the restaurant, I'd schedule a seating time nearer to 7:15 p.m. to give yourself plenty of time to enjoy your courses.
Keep in mind that no matter what time you eat dinner, you are allowed to return to the restaurant to view the fireworks. For example, if your dining time is 6:00 p.m. and you receive your check at 7:30 p.m., you may want to leave the restaurant and visit the shops inside the Contemporary before returning to the restaurant's check-in desk and securing re-admittance to visit the observatory deck.
Personally, I recommend a reservation time around 5:30 p.m. that time of the year, allowing you to enjoy the view both during daylight hours and to watch the sunset over the Grand Floridian resort. You can hop on the monorail to visit the other resorts on the line while you're waiting to return to the observation deck later in the evening.
Wishing y'all a magical vacation!
Whitney