An initiative by the William Pomeroy Foundation to get everyone involved in cleaning up historic markers around the country. Officially, this day is the last Friday of every April, no registration required. There is a video to explain how to properly care for the historic marker you've chosen to clean embedded at the bottom of the page (the YouTube link also listed below). Can be integrated into a classroom (even a remote classroom) to engage students in maintaining history and growing personal connections to the historic locations in their area.
Historic Marker cleaning videos https://www.youtube.com/watch?v=2eYOBrlDBgM and the marker day in particualr