day=`date +"%Y%m%d"`
month=`date +"%m"`
year=`date +"%Y"`

days_in_month()                # maximum days of every month
{
  case $month in
    09|04|06|11) maxdays=30 ;;
    01|03|05|07|08|10|12) maxdays=31 ;;
    02) if (( ("$year" % 400) == "0" )) || (( ("$year" % 4 == "0") && ("$year" % 100 != "0") )) ; then
          maxdays=29
        else
          maxdays=28
       fi
  esac
}

check_end_of_month_year()        # check if it will be end of month/year
{
  if [ $day_temp -gt $maxdays ]; then # reached end of month
    day_temp=01
    month=$[$month+1]
    if [ $month -gt 12 ]; then       # reached and of year
      month=1
      year=$[$year+1]
    fi
    new_month=yes            #
    days_in_month            # calculate max days in new month
  fi
  month=`printf "%02d" $month`
}

days_in_month                   # how many days does current month have
new_month=no                  # parameter to indicate new month or not
day[0]=`date +"%e"`            # put date of today in day0
FILE0=$day"_US.gif"            # real filename for FILE0 download

# ######################################
# ------- I think the source below can be smaller -------
# ######################################

j=1
for((i=0;i<5;i++))
do
let day[$j]=$[${day[$i]}+1]
let j=j+1
done

for((i=1;i<6;i++))
do
if [ $new_month = yes ]; then
  day_temp=$[$day_temp+1]        # day_temp will be used in check_end_of_month_year
else
  day_temp=${day[$i]}
fi
check_end_of_month_year            # check if it will be end of month/year
day[$i]=`printf "%02d" $day_temp`        # day1 = daytemp but with leading zero

#echo ${day[$i]}

if [[ $i -eq 1 ]];then
FILE1=$year$month${day[$i]}"_US.gif"        # real filename for FILE1 download
#echo $FILE1
elif [[ $i -eq 2 ]];then
FILE2=$year$month${day[$i]}"_US.gif"        # real filename for FILE1 download
#echo $FILE2
elif [[ $i -eq 3 ]];then
FILE3=$year$month${day[$i]}"_US.gif"        # real filename for FILE1 download
#echo $FILE3
elif [[ $i -eq 4 ]];then
FILE4=$year$month${day[$i]}"_US.gif"        # real filename for FILE1 download
#echo $FILE4
elif [[ $i -eq 5 ]];then
FILE5=$year$month${day[$i]}"_US.gif"        # real filename for FILE1 download
#echo $FILE5
fi
done