Don't feel left out. It's one that escaped my notice too. Maybe that's because it's really hard to find on any calendar.
As an avid letter of letters, cards and notes I didn't know about the date until a few weeks ago. I received a packet of cards from our supplemental insurance provider and the accompanying letter said that the notecards were to celebrate "Forget-Me-Not" Day. Recipients were invited to send them to others.
Curious about the origins of this "holiday" I checked online for more info. According to one site, it was created as a day to remember and get in touch with friends and family you care about and, perhaps, don't see often. Another noted that some people used it to memorialize a loved one who has passed away.
The National Day Calendar website, stated that it was created in 1921 to remind Americans about the sacrifices of returning soldiers. The date was said to have raised funds for services where there were none. (The website didn't specify more details.)
Regardless of its origins, it's an easy day to celebrate. If you're not a letter writer, email or call friends and family to let them know you're thinking of them. Of course, where possible, getting together with them is always better.
Never underestimate the power of taking the time to let someone know that you're thinking about them. It can mean more than you think it does.
“I’ve learned that people will forget what you said, people will forget what you did, but people will never forget how you made them feel.” Maya Angelou