calendar-month-name-array

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

["January" "February" "March" "April" "May" "June" "July" "August" "September" "October" "November" "December"]

Documentation:
Array of capitalized strings giving, in order, the month names.
If you change this without using customize after the calendar has loaded,
then you may also want to change `calendar-month-abbrev-array'.

You can customize this variable.