calendar-nth-named-absday is a compiled Lisp function in `calendar.el'.

(calendar-nth-named-absday N DAYNAME MONTH YEAR &optional DAY)

Absolute date of the Nth DAYNAME after/before MONTH YEAR DAY.
A DAYNAME of 0 means Sunday, 1 means Monday, and so on.
If N>0, return the Nth DAYNAME after MONTH DAY, YEAR (inclusive).
If N<0, return the Nth DAYNAME before MONTH DAY, YEAR (inclusive).
DAY defaults to 1 if N>0, and MONTH's last day otherwise.