I am one of the UW residents.
We know how difficult it is for applicants to make it to interviews during the busy application process. The idea behind the two socials is to give the applicants two options. The Friday night social is at the program directors house with the Chief residents primarily. The Saturday night social is with all the residents, significant others, etc... (no attendings) and it is usually a larger gathering at a bar/restaurant. Both socials are a lot of fun and I really enjoyed them when I interviewed here.
There is no penalty for not going to both socials. They are just there for you to get to know us and for us to get to know you.