/usr/bin/calendar
The calendar utility checks the current directory for a file named calendar and displays lines that begin with either today's date or tomorrow's.
/System/Library/CoreServices/SetupAssistantPlugins/Calendar.flplugin/Contents/MacOS/Calendar
/System/Applications/Calendar.app/Contents/MacOS/Calendar
Manages and keeps track of events and appointments. You can keep track of important dates, manage schedules and set reminders to ensure you don't miss important deadlines or events.
/System/Applications/Calendar.app/Contents/Resources/Calendar.docktileplugin/Contents/MacOS/Calendar