Name: Just Calendar
Mac Platform: Intel
OS version: 10.10
Processor type(s) & speed: 64-bit
RAM minimum: NA
Video RAM: NA
Simply drag to Application's folder and double click. A new widget will appear as being available in Notification Center.
If you remember the calendar widget for Dashboard, then you should be familiar with this. It is a Notification Widget that provides you with a simple monthly calendar. The same developer has some other nice widgets too for unit and currency conversion (Convertos and Currencier) which are both available here.
Tested in Yosemite and El Capitan.
Mac App Store: https://itunes.apple.com/us/app/just-calendar/id934523970?mt=12