this script will mysqldump your db daily, weekly and monthly. and it will only keep three of them, while doing the fourth backup, it will delete the first one first.

mysqldump-cron.sh

#!/bin/bash

#author Lyman Lai
#date  2013-12-30
#source http://002.yaha.me/item/22728a58-c967-46d5-93eb-2649d684a9aa/

MYSQL_USER="yourname"
MYSQL_HOST="localhost"
MYSQL_PASSWORD="yourpassword"
MYSQL_BIN="$(which mysql)"
MYSQLDUMP_BIN="$(which mysqldump)"

STORE_FOLDER="."

TODAY=$(date +"%Y-%m-%d")
DAILY_DELETE_NAME="daily-"`date +"%Y-%m-%d" --date '3 days ago'`
WEEKLY_DELETE_NAME="weekly-"`date +"%Y-%m-%d" --date '3 weeks ago'`
MONTHLY_DELETE_NAME=`date +"%Y-%m-%d" --date '3 months ago'`

for db in 'db_name_1' 'db_name_2' 'db_name_3'
do
  THE_PATH=$STORE_FOLDER/$db
  echo  $DAILY_DELETE_NAME
  if [ ! -d "$THE_PATH" ]; then
    mkdir -p "$THE_PATH"
  fi
  echo $THE_PATH/$DAILY_DELETE_NAME.sql.gz
  rm -rf $THE_PATH/$DAILY_DELETE_NAME.sql.gz
  rm -rf $THE_PATH/$WEEKLY_DELETE_NAME.sql.gz
  rm -rf $THE_PATH/$MONTHLY_DELETE_NAME.sql.gz

  #daily
  $MYSQLDUMP_BIN -u $MYSQL_USER -h $MYSQL_HOST -p$MYSQL_PASSWORD $db | gzip -9 > $THE_PATH/daily-$TODAY.sql.gz

  #weekly
  if [ `date +%u` -eq 7 ];then
    cp $THE_PATH/daily-$TODAY.sql.gz $THE_PATH/weekly-$TODAY.sql.gz
  fi

  #monthly
  if [ `date +%d` -eq 25 ];then
    cp $THE_PATH/daily-$TODAY.sql.gz $THE_PATH/monthly-$TODAY.sql.gz
  fi
done