Kicking off #FreeSoftwareAdvent (thanks,
@neilNeil Brown), I'll open with remind(1)
While it took several articles and a couple attempts before I switched over to using it, once you taste the power of what it can do, it's hard to go back to less-capable calendaring tools.
While the classic "garbage day is on Thursday unless there was a holiday earlier in the week, in which case it moves back to Friday" scenario is a nice little demo of its power, one of the best examples from my daily use is the kids' school calendars:
• the teen has an A/B schedule which doesn't mesh nicely with calendar days, week-days, etc
• similarly, our elementary-age kiddo has a 4-day cycle schedule for her "specials" class
But remind's nonomitted() function makes quick work of both of those, taking into consideration weekends, the school holidays, and using PUSH/POP directives for high-school testing days that impact his A/B schedule but not her 4-day cycle. I've never encountered another calendar that handled all the edge-cases with so little effort.
It's a little rocky interchanging with other calendars (you have to use rem2ics to create .ics files to share, and pulling in others' iCal is non-trivial and doesn't seem to maintain the fidelity of remote events).
But otherwise, this runs a great deal of my life schedule.