Hi Asher,
Extra Magic Hours are a great perk for Walt Disney World resort guests. During Extra Magic Hours, Walt Disney World resort guests can enjoy extra time in a Disney theme park either before the park opens or after it closes. Be sure to check the theme park hours for the day you are going to confirm which theme park offers Extra Magic Hours on the day you are going. You do need to have valid theme park admission for the day on which you will take advantage of Extra Magic Hours.
I am not sure how many days of Disney theme park admission you have, but remember that the price you pay per day goes down with each day you add. So, for example, if you would need to add a day to a two-day ticket to have three days of admission in order to attend Extra Magic Hours on your first night, that would be more expensive that if you were adding an eighth day onto a seven-day ticket.
If you do not want to add another day onto your theme park tickets, I have another suggestion for you. You mention that you are staying at
Disney's Polynesian Resort, which is such a beautiful resort. The tropical theme is lush and relaxing. You can get a great view of the Magic Kingdom's Wishes Nighttime Spectacular from the beach at Disney's Polynesian Resort. Once you arrive and settle in, head to the beach, grab a snack or a drink at the Barefoot Pool Bar, and enjoy the amazing fireworks show.
I hope that this information is helpful, Asher! Have a great time planning your vacation, and please do not hesitate to ask if you have any further questions.