date -d @$(( $(date -d $(date -d @$(( $(date +%s) + 2678400 )) +%Y%m01) +%s) - 604800 )) +%d

$ cal | awk '/^ *[0-9]/ { d=$1 } END { print d }'
25

$ cal
    December 2016
Su Mo Tu We Th Fr Sa
             1  2  3
 4  5  6  7  8  9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30 31

#!/bin/bash

# print last Sundays for 2016 and 2017

for y in {2016..2017}
do
  for m in {1..12}
  do
    first_day_of_month=$y/$m/1
    days_in_month=$(date +%d -d "$first_day_of_month month day ago")
    day_of_week_of_last_day=$(date +%w -d "$first_day_of_month month day ago")
    last_sunday=$(date -d "$first_day_of_month $days_in_month day $[1+$day_of_week_of_last_day] day ago")
    echo $last_sunday
  done
  echo
done

# print last Sunday for current month

first_day_of_month=$(date +%m/1)
days_in_month=$(date +%d -d "$first_day_of_month month day ago")
day_of_week_of_last_day=$(date +%w -d "$first_day_of_month month day ago")
last_sunday=$(date -d "$first_day_of_month $days_in_month day $[1+$day_of_week_of_last_day] day ago")
echo $last_sunday

Sun Jan 31 00:00:00 GMT 2016
Sun Feb 28 00:00:00 GMT 2016
Sun Mar 27 00:00:00 GMT 2016
Sun Apr 24 00:00:00 BST 2016
Sun May 29 00:00:00 BST 2016
Sun Jun 26 00:00:00 BST 2016
Sun Jul 31 00:00:00 BST 2016
Sun Aug 28 00:00:00 BST 2016
Sun Sep 25 00:00:00 BST 2016
Sun Oct 30 00:00:00 BST 2016
Sun Nov 27 00:00:00 GMT 2016
Sun Dec 25 00:00:00 GMT 2016

Sun Jan 29 00:00:00 GMT 2017
Sun Feb 26 00:00:00 GMT 2017
Sun Mar 26 00:00:00 GMT 2017
Sun Apr 30 00:00:00 BST 2017
Sun May 28 00:00:00 BST 2017
Sun Jun 25 00:00:00 BST 2017
Sun Jul 30 00:00:00 BST 2017
Sun Aug 27 00:00:00 BST 2017
Sun Sep 24 00:00:00 BST 2017
Sun Oct 29 00:00:00 BST 2017
Sun Nov 26 00:00:00 GMT 2017
Sun Dec 31 00:00:00 GMT 2017

Sun Dec 25 00:00:00 GMT 2016