Highlight dates in cal output with awk.

Question

Lately I've stumbled across a problem (read: I had a stupid idea and now really want to make it happen) where I wanted to highlight significant dates in the terminal calendar cal. now I have a "working solution" which highlights the dates, however it's got two major issues:
1) It's ugly, as in unreadably ugly which can probably be improved with more expertise.
2) It requires me to add another pipe and awk statement for every date I want to highlight
what I was hoping to find out is if I could somehow incorporate a loop to run through the pipe with different values each time, it seems simple in theory but in practice it falls apart on me, so is it at all possible?
My command currently looks like this:
cal -y | awk -v month="`date +%m`" -v day="`date +%e` " '{m=int((NR-3)/8)*3+1; for (i=0;i<3;i++) {t=substr($0,1+i*22,20) " "; if (m+i==month) sub(day,"\033[0;31m&\033[0m",t);} print t[0],t[1],t[2];}' | awk -v month="10" -v day="31 " '{m=int((NR-3)/8)*3+1; for (i=0;i<3;i++) {t=substr($0,1+i*22,20) " "; if (m+i==month) sub(day,"\033[0;31m&\033[0m",t);} print t[0],t[1],t[2];}' | awk -v month="04" -v day="25 " '{m=int((NR-3)/8)*3+1; for (i=0;i<3;i++) {t=substr($0,1+i*22,20) " "; if (m+i==month) sub(day,"\033[0;31m&\033[0m",t);} print t[0],t[1],t[2];}' | awk -v month="02" -v day="14 " '{m=int((NR-3)/8)*3+1; for (i=0;i<3;i++) {t=substr($0,1+i*22,20) " "; if (m+i==month) sub(day,"\033[0;31m&\033[0m",t);} print t[0],t[1],t[2];}'
and I was hoping to figure out a way to achieve the form:
cal -y | for i in `date %m%e` `cat FileWithDates`; do awk -v month="`sed command to extract int before the first whitespace from $i`" -v day="`sed command to extract int from after first whitespace in $i` " '{m=int((NR-3)/8)*3+1; for (i=0;i<3;i++) {t=substr($0,1+i*22,20) " "; if (m+i==month) sub(day,"\033[0;31m&\033[0m",t);} print t[0],t[1],t[2];}' |;done; echo
or really anything where I don't have to tack on an awk statement to the end of this thing for every date I want to highlight when it should be possible to get the same from a file. Maybe I'm just addressing this problem wrong... Either way any help would be appreciated!
I'll readily admit to not knowing enough about awk, so if I'm missing an easy way to fix this that involves changing the awk statement(s) I'm more than open to that too.

Answer 1

#!/usr/bin/env bash
# This is example code I used to test.
# You get to build it anyway you want.
echo "##############################"
echo "#### create FileWithDates ####"
cat >/tmp/FileWithDates <<EOD
10 31
4 25
2 14
8 4
EOD
cat /tmp/FileWithDates # I needed somewhere to put my example file
echo "##############################"
# This is the part of the script you are interested in
cal -y | awk -v month="$(date +%m)" -v day="$(date +%e) " '
BEGIN {
# handle the current month and day
# The +0 makes sure the value is treated
# as a number, and not a string
max=0
sub(/^0/,"",month)
months[max] = month + 0
days[max] = day + 0
max++
NR == FNR {
# collect the entries from FileWithDates
# NR will only equal FNR for the first
# file being read, so it is essential that
# FileWithDates is the first file name
# specified on the awk command line.
# Again, +0 forces the value to be a
# number, not a string.
months[max] = $1 + 0
days[max] = $2 + 0
max++
next # Do not fall into next section
# We will only get here after fulling reading FileWithDates.
# The use of - on the command line below tells awk to read from
# stdin, which in our case should be the output from cal -y
m = (int((FNR-3)/8) * 3) + 1
for (i=0; i<3; i++) {
# extract the week of the month. I have put a space
# before the week and a space after the week. This
# does change the cal -y output slightly, but it makes
# the code easier, as I can depend on every day having
# a space before and after, which makes getting a
# unique match much easier.
t = " " substr($0,1+i*22,20) " "
for(j=0; j < max; j++) {
# I added this for(j...) loop to check each of the
# FileWithDates and the current month and day values
# against each week of a month.
if (m+i == months[j]) {
# I am just marking the days to be colored
# with :nn@, as I do not want the
#  [0;31m to be confused for the
# 31st of the month.
sub(" "days[j]" ",":"days[j]"@",t)
# Replace : with space  [0;31m
# Replace @ with  [0m space
gsub(/:/,"  [0;31m",t)
gsub(/@/," [0m ",t)
# Print each week of the month for this output row
print t[0],t[1],t[2]
} ' /tmp/FileWithDates - # the - is essential for reading stdin