I don't know if this is the most efficient way to do this, but you could:
$date = ($numWeek-1) * 604800;
switch ($dayName)
{
case "Sunday"
$date += 86400;
break;
case "Sunday"
$date += 172800;
break;
case ... //you get the point
}
$date += mktime(0, 0, 0, 0, 0, $year);
//You now have your time stamp for the day just use date() to get what you need