Chandler is an open source organizer which lets you create and save notes and dates in your calendar.
The interface is a little busy, but it really seems to have everything you need. The quick entry section at the top, which you could mistake for a search bar, lets you create notes quickly. Detail on your notes is shown on the right side of the interface. Chandler also displays a calendar, which you can browse through to see notes for each day.
In place of tags, Chandler has what it calls collections. These work more or less the same way and have plenty of specific options, but you'll need to categorize your appointments well to make them work properly.
Chandler allows you to easily import and export information, mostly as .ics files, and you can share your notes with others, subscribe to others' calendars, have them subscribe to yours, and publish your data to a third-party website all via a Chandler Hub account. You'll have to set this up separately, but it's free.
Despite feeling strangely old-fashioned, Chandler is a very solid, complete note-taking and organization app.