findLastTagPrecedingDate.sh

#!/bin/bash

if [ "$#" -ne 2 ] ; then
	echo "Usage: `basename -- $0` date tagFilter"
	echo "eg: `basename -- $0` 20201225 TAG_SUBSTRING_TO_MATCH" 
	exit 1
fi

DATE=$1
SEARCH=$2

YEAR=${DATE:0:4}
MONTH=${DATE:4:2}
DAYS=${DATE:6:2}


git for-each-ref --sort=creatordate --format '%(refname) CreateDate=%(creatordate)' refs/tags |
	grep ${SEARCH} |
	awk -F'CreateDate=' -v YEAR=${YEAR} -v MONTH=${MONTH} -v DAYS=${DAYS} '
BEGIN{
   m=split("Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec",d,"|")
   for(o=1;o<=m;o++){
      months[d[o]]=sprintf("%02d",o)
   }
   before = mktime(YEAR " " MONTH " " DAYS " 00 00 00")
}
{
  split($2,time," ")
  date = (time[5] " " months[time[2]] " " time[3] " 00 00 00 ")
  timestamp = mktime(date)
  if ( timestamp < before ) {
    split($1,tagElements,"/")
    print tagElements[3]
  }
}
' | tail -1l

Find the last tag in a git repo which precedes a date and matches a search string
Needs to be run in the same directory as the repo (or amend to take git repo dir as an extra argument)