calendar-month-abbrev-array is a variable defined in `calendar.el'.
Its value is

["Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"]

Array of capitalized strings giving the abbreviated month names.
The order should be the same as that of the full names specified
in `calendar-month-name-array'. These abbreviations are used in
the calendar menu entries, and can also be used in the diary
file. Do not include a trailing `.' in the strings specified in
this variable, though you may use such in the diary file. By
default, each string is the first ``calendar-abbrev-length'
characters of the corresponding full name.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.