The USA celebrate National Memo Day on May, 21 2018. A memo is a note that reminds us of something important. That memo can be digital, like an alarm on your smartphone, and written on a sheet of paper. Often colorful adhesive post-its are used to scribble memos on them. They are sent via e-mail frequently, too.
It is unclear where this holiday comes from but the way to celebrate is obvious: Write, send or read a memo. On National Memo Day one can remind himself of a certain topic using a memo or inform other people about something important they should not forget. You could even draft a memo for the next upcoming National Memo Day.