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

(calendar-string-spread STRINGS CHAR LENGTH)

Concatenate list of STRINGS separated with copies of CHAR to fill LENGTH.
The effect is like mapconcat but the separating pieces are as balanced as
possible. Each item of STRINGS is evaluated before concatenation so it can
actually be an expression that evaluates to a string. If LENGTH is too short,
the STRINGS are just concatenated and the result truncated.