calendar-date-string is a compiled Lisp function in `calendar.el'.

(calendar-date-string DATE &optional ABBREVIATE NODAYNAME)

A string form of DATE, driven by the variable `calendar-date-display-form'.
An optional parameter ABBREVIATE, when non-nil, causes the month
and day names to be abbreviated as specified by
`calendar-month-abbrev-array' and `calendar-day-abbrev-array',
respectively. An optional parameter NODAYNAME, when t, omits the
name of the day of the week.