Final answer:
National Stranger Things Day is celebrated on November 6th, chosen based on the disappearance of the character Will Byers in the show. Fans mark this day with activities related to the series.
Step-by-step explanation:
National Stranger Things Day is celebrated on November 6th. This day was chosen by fans and the creators of the show to commemorate the date in 1983 when the fictional character Will Byers disappeared in the first season of Stranger Things, which is also the event that sets the entire series in motion.
It's an unofficial holiday where fans celebrate the show by engaging in various activities related to Stranger Things, like binge-watching the series, dressing up as characters, and discussing fan theories.