Since it also is on Cinco de Mayo, there are some joint events going on. This town likes an excuse to party. Here’s how I find out about things going on: twitter, Eventbrite, Google Boston.
An example is Sara from EventBrite.com: https://twitter.com/#!/BriteBoston/status/198430042609553408
I’ve found a listing of Kentucky Derby viewing places in town, so there are several to pick from.