November 19th is statistically the wettest day of the year in Seattle, or at least, the day it has rained on the most in Seattle history, going back to 1893.
Over the past 115 years, it has rained on this date a whopping 86 times, not counting 2008. That's a 75% chance, or basically it rains on this date three out of every four years. The next wettest dates are November 15th (80), December 1st (79) and January 13th (78).
(Side personal note, for those who read my discussion last year, I was bummed because January 13th is my birthday and it used to be second wettest, which I thought was kind of cool, but it's been dry the past few years to fall down the chart.)
But this year, it appears November 19th will be that rare occurrence where it'll be dry all day. Guess you better plan your weddings for the 18th or 20th in 2009-2011? :) (OK, that's not how statistics really work, but if you believe in the law of averages...)
The driest dates are in late July/early August right around the peak SeaFair events (which is not coincidental. The organizers had done their weather research when selecting the dates.)
There is a three-way tie for driest dates: July 30, August 1st and August 4th, each with just 9 instances of rain in the past 115 years, or a paltry 7.8% chance of getting rained on in any given year. (Or, put another way, it'll rain on those dates about once in every 13 years.)
Another quirk -- did you know July 4th is statistically the most likely day to get rain in July? It's only 33 times, but it's three ahead of second place (July 3rd; 30 instances).
You can check the entire chart at this link.