When I first heard the term 'WordPress contributor day,' I imagined it was a day for all the volunteers and speakers to get together. I'm not sure to what purpose; I didn't think that far. But in case you did have any strange notions like me, here's what it is. A contributor is anyone who wants to contribute to WordPress.
When I learned this, I thought I would have to know how to code the hard stuff, and I don't have that level of proficiency, but it wasn't the case at all. l. There are a dozen or so ways you can be a contributor.