This app was mentioned in 2 comments, with an average of 3.00 upvotes
For android users, Event Logger is a similar app that works offline.
And if you want one that lets you add notes to events, there's another app also called Event Logger, it just takes a bit of time to setup in the first place.
There's no calendar integration, but an app I made a while ago might work for this
https://play.google.com/store/apps/details?id=com.cr5315.eventlogger