RAZORalex,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<script>
function fn(a, b, c) {
c = new Date(a, b - 1, c);
b = (new Date(a, 0, 1)).getTime();
a = [7, 1, 2, 3, 4, 5, 6][c.getDay()];
c = c.getTime();
return 1 + Math.ceil(((c - b) / 36E5 / 24 - a) / 7)
};
document.write(fn(2015,12,5));
</script>
</body>
</html>