Any good FLOSS day planner ?

Hello,

I've been looking for a while for a free day planner. While there are a lot of proprietary ones, there seem to be no good free option apart from traditional calendar apps.

I'm looking for something like Structured, and I find it difficulted to believe that there are millions of users of proprietary day planners and still seem to be no good alternative. Unfortunately I am no developer.

Do you know or use any good free day planner app ?