For context I've been using aerc as my email client for a while now, and was looking for something similar for calendars/tasks myself

I've tried:

• calcurse: fine but clunky, also a bit difficult to set up. The most mature option and probably the best one available, but I just couldn't get used to the interface
• calcure: similarly clunky interface, glitchy/blinking rendering to the point of being headache-inducing, lacking features (couldn't figure out how to look at all event attributes?)
• khal: limited in features (compared to calcurse) and slow when there are a lot of events (even when it's only 2-3 per day), also there are some rendering bugs sometimes. Probably the most intuitive and clean interface of all, and good scripting opportunities.
• gcalcli: only Google Calendar (I also need support for arbitrary CalDAV), didn't investigate further
• plann: no TUI as such, just CLI

A couple weeks ago I've decided to start writing my own. It's still very much a hacky WIP but I'll update in this thread if I ever decide to publish it. In the meantime, I hope one of the above works for you!