It is the third annual public domain day in the United States. January 2019 was the first time in a long time that anything covered by copyright in the US entered the public domain. This year makes 3 years of stuff finally making it to the point that they can be freely distributed, built upon, reworked, or used however anyone wishes.
One cool option for using public domain material this year is to check out the Gaming Like Its 1925 game jam over on itch.io. This game jam is hosted by Some very awesome people who are interested in promoting the public domain and working to make copyright work for the people once again. There are prizes available and just a lot of fun to be had there for all interested.
And just like every year, the Duke Law School has published an article highlighting many of the works that have entered the public domain at midnight last night. There is a ton of books, movies, music, non-fiction, and a lot more that are now available for anyone.