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 118 years, it has rained on this date a whopping 88 times, including 2010 since 0.01" had fallen. 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 (82), and December 1st (80).
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 tie for the driest dates: July 30 and August 4th, each with just 9 instances of rain in the past 117 years, or a paltry 7.7% chance of getting rained on in any given year. (Or, put another way, it'll rain on those dates about once 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 two ahead of second place (July 3rd; 31 instances).
You can check the entire chart at this link.