You can use the date function. I'm using strtotime to get the timestamp to that day ; there are other solutions, like mktime, for instance.

For instance, with the 'D' modifier, for the textual representation in three letters :

$timestamp = strtotime('2009-10-22');

$day = date('D', $timestamp);
var_dump($day);
You will get :

string 'Thu' (length=3)
And with the 'l' modifier, for the full textual representation :

$day = date('l', $timestamp);
var_dump($day);
You get :

string 'Thursday' (length=8)
Or the 'w' modifier, to get to number of the day (0 to 6, 0 being sunday, and 6 being saturday) :

$day = date('w', $timestamp);
var_dump($day);
You'll obtain :

string '4' (length=1)