For us, we wanted to steer clear of major holidays because people are usually busy with family which means lower attendance. Look through a list of all the US holidays and observances (like this one here) or just be listening for relevant cultural things going on (Shark Week, royal weddings, etc).
At the end of the day, your guests won’t really care that much what they’re celebrating. They’ll just be content with food, drinks, and games. Only the detail-oriented friends will pick up on the time you put into your theme, so just decide what’s most important to you: Slappin’ the words “Cinco de Mayo Party” on the evite and calling it a day (if that’s the case, stop reading this post now), or going all-out with sombreros, themed food, photo booths, etc. (if that’s you, keep readin‘!)
Another easy way I chose to incorporate the theme was to use my chalkboard table! I drew a picture of a shark and an appropriate hashtag for my tech-savvy guests to use throughout the night if they chose to do so.