Version 5 Unix: https://minnie.tuhs.org/cgi-bin/utree.pl?file=V5/usr/source/s1/cal.c
char dayw[]
{
" S M Tu W Th F S"
};
Two decades later, 4.4BSD had
char *day_headings = " S M Tu W Th F S";
I find it interesting that the cal/ncal manual from a Ubuntu systems
says "The output of the cal command is supposed to be bit for bit
compatible to the original Unix cal command, because its output is
processed by other programs ..."
