RAZORalex,
 
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<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>
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