When is the next grindable event ?

According to this guide that was posted here a little while back:

https://docs.google.com/spreadsheets/d/1qvxLU407QwiFaCvItqR16SqqAVlLD5u5nBzY_bCFYvs/edit#gid=1231775758

The next event with Knight Medals will be this one in April:

https://grandorder.wiki/GUDAGUDA_Meiji_Ishin

If you can wait till September, Nerofest lottery box have tons of it