The Seattle/King County Convention and Visitors Bureau (206/461-5840, www.seeseattle.org [1]) can provide details on year-round events, starting with a big fireworks show from the Space Needle [2] on New Year’s Eve and lots of partying downtown.
Links:
[1] http://www.seeseattle.org
[2] http://www.moon.com/destinations/washington/seattle/sights/seattle-center-and-belltown/space-needle
[3] http://www.moon.com/destinations/washington/seattle/entertainment-and-events/festivals-and-events/winter
[4] http://www.moon.com/destinations/washington/seattle/entertainment-and-events/festivals-and-events/spring
[5] http://www.moon.com/destinations/washington/seattle/entertainment-and-events/festivals-and-events/summer