Hiya Nikki!
Welcome to planDisney! I can't believe I am already thinking about New Year's Eve while enjoying this spring day! I have never had a chance to enjoy New Year's Eve in the Parks but last year's events showed me that it is something I have to experience at some point.
It is not yet known what the official start time for the evening parties at
EPCOT will be. I do know that each of the 11 countries in the World Showcase has its own New Year's Countdown with the order being Japan at around 11 AM EST, followed by China, Morocco, Italy, Germany, Norway, France, the United Kingdom, Canada, the United States, and then Mexico. Most likely there will be events and activities including music throughout the day. Last year, the DJ dance parties were announced on social media and details were sparse. The people I know who attended last year said it was an incredibly fun event with memorable moments. One party they especially loved was the giant dragon and flames at the China Pavilion. Most of the details about New Year's Events in EPCOT last year ended up appearing on social media sites such as Instagram.
The Disney Parks Blog may post some details so I would keep an eye out for that. I suspect that by ensuring you arrive by midday you will find some festivities and music. Most guests found sunset to be the prime dance party time which should be around 5:30 PM EST. EPCOT has a large capacity but I would still try to come early enough before they stop guests from entering as they reach the capacity limits.
Just a heads up, there will most likely be fireworks occurring twice on New Year's Eve at 6:30 PM and 11:40 PM. Since this will be a busy time, grab a spot to enjoy the show at least two hours prior. I would also consider staying at a Resort in the area such as
Disney's BoardWalk Inn,
Disney's Beach Club Resort, or
Disney's Yacht Club Resort. This way, when the festivities are over you can just walk to your room rather than waiting for other means of transportation which is sure to have long lines.
I hope this helps and you have a fabulous time!
Happy Planning!
-Nimmy