i'm terrible at keeping up with hashtag events but i remember i used to keep track of this calander organized by Abelle Hayford iirc that was incredibly useful
the link should take it to January 2021 but it's the one year where all the hashtags and other events are fully filled out since idk who maintains it now or if it's being kept up with
Still it's got everything laid out from repeat events like Portfolio Day to more monthly/annual stuff like Mermay and Nativetember
Personally I've kinda backed off of doing monthly challenges since I tend to burnout easily and keeping up with doing fully illustrations everyday for like 30 days is a lot but i almost always try to pop into Portfolio Day when i can along with Blacktober and DrawingWhileBlack