LyxCalendar

Lets you create events and highly customizable reminders, and won't sleep on you because it's using a foreground service which is always active, so it's impossible to not get notified.

Also supports 'flexible events', which work ideally for events that don't necessarily have to be completed on a set date but rather every x days for example. If you skip a day and set it to done, the next event will automatically created based on the time you completed it.