source: vendor/bash/3.1-p17/examples/scripts.v2/cal2day.bash

#!/bin/bash
# cal2day - "parse" appropriate calendar output to match date number
#           with day name.
#
# usage: cal2day month day [year]
#
# ORIGINAL *TAG:33239 3:Dec 9 1997:0755:sh.d/cal2day:
#
# Obtained from usenet
#
# Converted to bash v2 syntax by Chet Ramey <chet@po.cwru.edu>

#1 PARSE OPTIONS
while getopts :dls _inst
do      case $_inst in
        (d)     format='%1d%.0s\n' ;;           # 0, 1, ..., 7
        (l)     format='%0.s%-s\n' ;;           # Sunday, Monday, ..., Saturday
        (s)     format='%0.s%-.3s\n' ;;         # Sun, Mon, ..., Sat
        esac
done
shift $((OPTIND-1))

#2 PARAMETER VALUES
((!$#)) && set -- $(date '+%m %d')
: ${format:='%0.s%-.3s\n'}
: ${1:?missing month parameter [1-12]}
: ${2:?missing day parameter [1-31]}

#3 CALCULATE DAY-OF-WEEK FROM DATE
cal $1 ${3:-$(date +%Y)} | gawk -FX '
BEGIN   { day="Sunday   Monday   Tuesday  WednesdayThursday Friday   Saturday"
          sub(/^0/, "", daynum)
          dayre="(^| )" daynum "( |$)"
        }
#NR==2  { print length($0) }
NR==1 || NR==2 \
        { next }
dayre   { if (match($0, dayre))
          {     #print RSTART, RLENGTH, substr($0, RSTART, RLENGTH)
                if (daynum<=9 || RSTART==1) RSTART-=1
                exit
          }
        }
END     { # 20/21 char width assumed
          printf format, RSTART/3, substr(day, RSTART*3+1, 9)
        }
' daynum=$2 format=$format -

exit 0