The Electrical Water Pageant starts each night at 9:00 at the Polynesian. The times don't vary due to time of year.
EXCEPT...if the fireworks at the Magic Kingdom start at 9:00, the EWP begins immediately after the fireworks end. Very cool from the Polynesian!
Someone posted that the new Wishes fireworks last around 20 minutes...much longer than the old ones. So if there are fireworks at 9 on the night you're interested, just add 20 minutes to the normal time for your resort.