Thanks for providing this heads-up, Sheryl.
Your message focused on U.S. records, which made me wonder if Fold3 also has non-U.S. military material. My brief visit to the site indicates that there are also some records for the U.K., Australia, and other nations (I found an English translation of a German report on German World War II plans and actions), so the free weekend may be worthwhile for our cousins from around the world.