Launch the Calendars app in your Applications folder.
For v10.7 Lion and earlier > Mac Basics: iCal (OS X Lion and earlier)
For v10.8 Mountain Lion, open Calendars. Then click Calendar Help from the Help menu.
In the future, please state which OS X is installed on your Mac.
Click the Apple () menu top left in your screen. From the drop down menu click About This Mac. The version is noted there.
Advice can depend on that information.