Interactive. Get creative! Post something on social media riddled with grammatical errors. Ask your audience to pick out as many as they can. Whoever is correct gets an English award of some kind. Or, simply give them a freebie!
Blogs. Some of the most popular online-writing tools create blog posts for this day dictating the history of Grammar Day, or by offering "Quick and Dirty Tips" to grammatically correct writing.
Repurpose. Post a listicle video entailing tips your audience can use or historical fun facts.
Hashtags to use today: #NationalGrammarDay, #GrammarDay, #Grammar, #GrammaticallyCorrect, #GrammarNazi, #GrammarPolice, #English, #CorrectingEnglish, #NationalDays.