By the way, the 23rd day of September, 2006 is a Saturday.
Looks like you have Text in A1, that will happen when you put the text in front of the date in the same cell.
If you only have the true date in A1, formated as you listed, then VoG II's formula will work.
Type 23/9 in cell A1 and hit enter. Excel will recognize it as a date and format it to the default date format setup on your machine.
Select A1 again, hit Ctrl-1.
Custom format as "ddd mm/dd/yyyy", without quotes.