вроде вот так:
<?php
$month=1;
$day=1;
$year=1966;
$ddelta = 0;
for($i=1969;$i>$year;--$i)
{
if($i%4==0){$y2d+=366;}
else{$y2d+=365;}
}
$monts = array(31,28,31,30,31,30,31,31,30,31,30,31);
if($year%4==0){$monts[1]=29;}
for($step=0;$step<($month-1);$step++)
{
$ddelta += $monts[$step];
}
$days = $y2d-$ddelta-$day+1;
$fl = $days % 7;
?>