Few points that cause discomfort in the calendar app:
- event colors could be displayed already in the preview, now only after clicking on a day,
- clicking on another day could display that day instead of closing the preview of the first one,
- deleting an event deletes it but does not close the modal (should) and does not refresh the main window (or does not remove the event from it), should,
- hover/tap on a day could also display a button “new event” with the date prefilled to the date of the clicked day,
- new event modal too narrow, not responsive to the window width,
- no default calendar, should be pre-filled (what i see as a discomfort can be regarded as a feature, true),
- date format and first day of week should be based on user’s preferences,
- selecting minutes in the sub-modal: up & down arrows should increase/decrease by 5 or even 15 minutes,
- in that same sub-modal the set/cancel buttons (and maybe even the arrow buttons < > ^ v ) should be larger (do i have big fingers?),
- description should accept styles,
- setting a start date and time should set the end date and time to the same value as any earlier is anyway not meaningful.
I hope these small tips help!