http://it.php.net/manual/en/function.date.php

con il parametro N ottieni il giorno della settimana.

N ISO-8601 numeric representation of the day of the week (added in PHP 5.1.0)
1 (for Monday) through 7 (for Sunday)

oppure w

w Numeric representation of the day of the week
0 (for Sunday) through 6 (for Saturday)