Edit in JSFiddle

function result() {
  var res = 0b0;
  for (var i = 0; i < 7; i++) {
    if (document.getElementById("id" + i).checked) {
      var dayOfWeek = document.getElementById("id" + i).value;
      res = res | dayOfWeek;
    }
  }
  document.getElementById("code").innerHTML = res;
}
result();

<div>
  <input type="checkbox" id="id0" name="Sunday" value="0b1" onclick="result()">Sunday</input>
  <input type="checkbox" id="id1" name="Monday" value="0b10" onclick="result()">Monday</input>
  <input type="checkbox" id="id2" name="Tuesday" value="0b100" onclick="result()">Tuesday</input>
  <input type="checkbox" id="id3" name="Wednesday" value="0b1000" onclick="result()">Wednesday</input>
  <input type="checkbox" id="id4" name="Thursday" value="0b10000" onclick="result()">Thursday</input>
  <input type="checkbox" id="id5" name="Friday" value="0b100000" onclick="result()">Friday</input>
  <input type="checkbox" id="id6" name="Saturday" value="0b1000000" onclick="result()">Saturday</input>
</div>
<div style="padding: 10px; font-size: 18px;">
  <span>Code:</span> <span id="code">0</span>
</div>