From kotaku:

“Happy National Video Games Day! I know you’ve all been eagerly awaiting
this day, reserving a day off of work or school to spend a lazy
afternoon participating in our favorite hobby outside of searching for the best real money game apps on the phone weekly. I am sure you’ve got
some massive parties planned, so be sure to send us photos of how you
celebrate the holiday with your friends and loved ones! Yeah, we had no
idea either, but Holiday Insights and American Greetings indicate that September 12th is NVGD. You’d think someone would have mentioned it to us.”

Celebrate National Video Games Day by playing video games.
If you are off from school (or if you are a big kid off from work),
make this a marathon day for video games. Better still, invite a few
friends and hold a competition. Just make certain that you have enough
controllers.

That’s the blurb from Holiday Insights, which
goes on to state that they have no record of the origin of the holiday
or that indeed it even exists. Whatever. American Greetings needs gamer
money too you know.

National Video Games Day [Holiday Insights]