I've never found a better reminder utility for the command line than Dianne Skoll's Remind.
I used Remind for years. I was reminded (ha!) of Remind today when Dianne announced a new version (I still subscribe to the mailing list). It's the first update in three years.
My favorite way to use remind was with a curses front end called
Wyrd. It's a powerful combination, but I haven't been able to find a way to download Wyrd recently so I've not been using either.
UPDATE: While writing this I found a fork on Github that I may try building.
Dianne is not a fan of Apple or Microsoft, and goes so far as to hobble the app when it's built on a Mac. I understand her point, but it's a little heavy handed, so I remove that bit from the code before building it myself.
A few resources: