Linux remind ( https://dianne.skoll.ca/projects/remind) is a sophisticated reminder application for linux, MacOS & windows subsystem for linux (will likely run under cygwin [not tested by author]). It keeps your private data on your own computer (not shared around the world as "training data" by Google or Apple or other online calendar thing)

. . and when I say "sophisticated", this sumbitch will allow you to set very intricate schedules with which to be reminded.

It can run in the background (daemon mode) and send you emails or rings the system bell [*BEEP*] when an event is upcoming in order to warn you in advance.

The program reads a single file (or folder full of ".rem" files) where the reminders are kept by the operator

It can remind you of single events:

REM Jan 30 2020 AT 14:30 MSG Doctor's appointment

It can remind you of important dates, with as distinctive input as you like. For example, the following entries in a .reminder file would all remind of the various Mondays:

REM Mon 1 MSG First Monday of the month  ## every upcoming Monday the first
REM Mon 1 Jan MSG First Monday in January      ## every upcoming Monday the first in every January to come
REM Mon 1 Jan 2020 MSG First Monday in January, 2020 ## _Only_ Monday, January the first in 2020

Got a kid's tee-ball schedule that begins in April? One line will remind you weekly UNTIL the season is done.

REM Wed UNTIL 07 MAY 2020 MSG Little Johnny's t-ball practice%

On Ms. Skoll's page, there are links to other software projects which magnify the functionality of remind; for instance, I have enabled a "calendar view" web page which I bookmarked in my browser (this makes it easier to see what's coming up). Some of these devs have offered facilities to import CalDav & .ics files (used by Google & iCal calendars) if you're looking to toss the yoke of the all seeing Google/Apple calamity.

This thing does way more than I've detailed (way more options than I'm ever gonna use) and is able to

. . . and if you've not figured it out by now, the program is written for the command line (but can be extended, as explained)

A few other "remind" resources (there are many others):
https://www.43folders.com/2005/02/24/guest-mike-harris-looks-at-remind
https://old.reddit.com/r/selfhosted/comments/15igvry/application_for_recurren
https://blog.thechases.com/posts/remind