Hi Krystle,
I think that 8:40 PM is a great time to have made your Advance Dining Reservation at
'Ohana if
Wishes nighttime spectacular is scheduled to take place at the Magic Kingdom park at 10:00 PM on that particular day.
If you wish to have a table by the window where you will be able to watch Wishes nighttime spectacular from, I would recommend that you aim to arrive at the restaurant slightly early for your dining reservation (say 8:15 PM). When checking in, be sure to let the Cast Member know that you would like a table by the window and that you are more than happy to wait a little longer until a table at the window becomes available.
Whilst a table by the window cannot be guaranteed, I have always found that Cast Members will do their very best to accommodate any requests that Guests may have.
An alternative idea and something that my family love to do when we visit the Walt Disney World Resort would be to enjoy your meal at 'Ohana and then go down to the beach at Disney's Polynesian Village Resort to watch Wishes nighttime spectacular. It really is a beautiful and relaxed setting to watch the fireworks from and as an added bonus, the soundtrack to Wishes nighttime spectacular is piped in.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma