make weekly backup indepentend of month

backup.sh

@@ -1,4 +1,5 @@
 #!/bin/bash
+## Create by Stefan Sterz <hi@sterzy.com> in 2017
 
 ## General Settings
 SOURCES=(/root /etc /home /boot)  # What folder should be saved?
@@ -141,7 +142,7 @@ fi
 
 # Create a special backup approx every week
 if [[ $WEEKDAY = 7 ]]; then
-  let WEEK=($DAY+6)/7
+  let WEEK=$($DATE +"%W")%5 # save past 5 weeks
   backup "weekly/$WEEK"
 fi