Some things I have heard of: GoreTober, SwordTober, Julycantrophy, FolktalesWeek (November). If you are part of a fandom there is also probably monthly challenge of OTP or something, and special dates (like fourth of may for Star Wars fans). Some trends are recurrent like ArtVsArtist, DTIYS, or six fanart (a.k.a the most uncreative prompt in the universe).
What I observed is trendy is making "This user is. . ." on Instagram. Or simply make art meme.
I personally do not like participating in a whole, but will ride on the hashtag of I happen to draw something relevant.