Itsycal is a tiny calendar for your Mac's menu bar. If you want, it will display your appointments and serve as a companion to the Mac Calendar app. You can move Itsycal in the menu bar just like the built-in Apple menu extras by holding down the command key and dragging.
FIX (sort of) for macOS 10.13: Itsycal localizes the calendar and icon so that they are expressed in the user's native language no matter where they are. This worked on macOS 10.12 and prior, but stopped working on macOS 10.13. This version restores this functionality for a number of languages until Apple fixes the underlying issue in macOS 10.13.