Over the last decade or so the internet has assigned pretty much every pop culture franchise or character their own official day. "Star Wars Day" is May the 4th, "Alien Day" is April 26th, "Back to the Future Day" is October 21st, etc. Today, June 9th, is "Bill & Ted Day", so you'll probably see people on social media, cast members from the movies, and the studio that owns the rights to Bill & Ted's Excellent Adventure, Bill & Ted's Bogus Journey, and Bill & Ted Face the Music, encouraging us to be excellent to each other.
The date is a reference to a joke from the first movie. If it doesn't click for you immediately, check out the clip below for the answer. Plus, enjoy trailers for all three Bill & Ted movies and the intro from their early 90s animated series.