org-agenda-format-date

org-agenda-format-date is a variable defined in `org-agenda.el'.
Its value is

org-agenda-format-date-aligned

Documentation:
Format string for displaying dates in the agenda.
Used by the daily/weekly agenda and by the timeline. This should be
a format string understood by `format-time-string', or a function returning
the formatted date as a string. The function must take a single argument,
a calendar-style date list like (month day year).

You can customize this variable.