A proposed addition: even if you don’t typically go on AO3 (or LJ/ff.net/other dedicated fanfic sites), or if (heavens forbid) it’s down that day, you can still participate! Do you read most of your fic on Tumblr? Do you have a particular person you follow whose posts you’re always excited to see because their fic is some of your favorite?
Use this day to reblog some of their work! Likes, like kudos, are excellent and good, but don’t carry nearly the same weight on this site. And while it’s rare (impossible?) to delete an AO3 kudos, many people tend to periodically clean out their Liked posts, meaning even that feedback can actually disappear off of a post. It’s not uncommon for an older piece of Tumblr fic to actually have fewer notes over time because of this.
Reblogs are wonderful. Reblogs are life. Reblogs are a way of saying, “I don’t want to forget this, and I want everyone who follows me to see how great it is as well!” If you’re intimidated by the thought of leaving comments, tags are a great way to sum up the reason you liked something, even if it’s just “adorable” or “what goobers” (both actual tags I’ve gotten before, which made me giggle in delight), or semi-coherent flailing (e.g., “yasssss”, “fdlskajfla”, “hnnngggh”, etc.). Fic writers tag-stalk every bit as much as artists do, I promise, and we love to see your reactions there.