I had this problem recently . A reminder wouldn ' t stop notifying me even after I hit complete on it and it would just pop again a few minutes or seconds later . First , Log out of and back into iCloud . That simple step has worked for some people . If that doesn ' t work , go back into your iCloud settings , uncheck reminders and calendars . Go to finder and go to ~ Library and delete the calendar folder . It will recreate itself pretty quickly . Go back into iCloud settings enable Calendars and Reminders . And your reminderscalendars will sync back to the mac . That stopped the reminders notifications from reappearing for me . I found this solution at 0 tstart 00 tstart 0 <| sep |> It ' s not my opinion , it ' s just what it is .