Any recommendations for a FOSS encrypted calendar that supports sync?

After a short realization of how disorganized I am, my girlfriend said "어 진짜 I can't. I can't do this". So at the risk of my 6 year relationship falling apart, does anyone know of a good calendar we can use together to plan out our days?

EDIT: I appreciate everyone's concerns, but my relationship is not actually at risk. I'm just looking for a good calendar we can share 🥰