When viewing your calendars, you are able to have the calendar names display either the full name or the abbreviation by toggling the "Abbreviate Calendars" setting.
To change your setting, click More in the upper right corner on the calendar page, and then Abbreviate Calendars.
If you are the Sys Admin on your account, you are able to edit the calendar abbreviations. Hover over your name in the upper right corner and click Company Settings.
Choose Calendars and update the abbreviation for each calendar as you wish.