Get weekday of DD/MM/YYYY

I thought I'd almost got here with this script

set ShotDate to do shell script "exiftool -s3 -DateTimeOriginal /Users/StudioA/Desktop/MJC_0025.JPG "

logShotDate

set theYear to (characters 1 thru 4 of ShotDate) as text

set theMonth to (characters 6 thru 7 of ShotDate) as text

set theDay to (characters 9 thru 10 of ShotDate) as text

set myDate to (theDay & "/" & theMonth & "/" & theYear)

set thedate to datemyDate

logthedate

weekdaymyDate

the date will show the day of the week in its format

(*date Thursday, 21 August 2014 00:00:00*)

but I can't then set the weekday to Thursday?

Result:

error "Can’t get weekday \"21/08/2014\"." number -1728 from weekday "21/08/2014"