It's a bit of a crap shoot, but the 405 is generally a better option than I-5. Do check the WSDOT site on the day as it only takes a tiny bit of roadwork to grind everything to a halt .

Busiest times, as you would expect, are weekday rush hours (roughly 7-9am, 4-6pm). If you don't mind taking a bit more time consider using hwy 99 - yes, it's an urban road with lots of (unsynchronised ) traffic lights but I've never seen gridlock on it.