If your column is called ORIGINAL_DATE, the syntax will be
date ( weekstart( ORIGINAL_DATE, 0, -1 ), 'DD-MMM') AS WeekStart
The date function will format it as DD-MMM. The weekstart function will get the start of the week, but the standard is monday. The third parameter of that functions will offset the date by -1 to send it to sunday.
You can get the full explanation on the function with the definition on the help file (copy below)
Hope this helps,
Returns a value corresponding to a timestamp with the first millisecond of the first date ( Monday ) of the calendar week containing date. The default output format will be the DateFormat set in the script. Shift is an integer, where the value 0 indicates the week which contains date. Negative values in shift indicate preceding weeks and positive values indicate succeeding weeks. If you want to work with weeks not starting midnight between Sunday and Monday, you may indicate an offset in days in weekoffset. This may be given as a real number indicating days and/or fractions of a day.
weekstart ( '2006-01-12' ) returns '2006-01-09' with an underlying numeric value corresponding to '2006-01-09 00:00:00.000'
weekstart ( '2006-01-12', -1 ) returns '2006-01-02' with an underlying numeric value corresponding to '2006-01-02 00:00:00.000'
weekstart ( '2006-01-12', 0, 1 ) returns '2006-01-10' with an underlying numeric value corresponding to '2006-01-10 00:00:00.000'