The date function is expecting the second parameter to be a timestamp. It doesn't recognize your date format. You'll need to use the mktime() function with the pieces of $date to get what you want.
It takes six parameters. They are, in order:
hours, minutes, seconds, month, day, year. It's used like this.
echo ("December 1, 2000 is a " . date ("l", mktime(0,0,0,12,1,2000)));
You should be able to use either a split() or a substr() to parse out the pieces of your date format.
Hope this helps.