How to choose a date and get the week and enable select with specific values

Can you help me, because I'm "breaking head" days:

I'm trying to get the date with document.getElementById , but just in this field Nothing happens! It will be because of the calendar ...

function escrever(el) {
    var isso = document.getElementById('calendSemana').value = el.value; 

    //var dias_semana = new Array("Domingo", "Segunda-feira", "Terça-feira", "Quarta-feira", "Quinta-feira", "Sexta-feira", "Sábado");
    //var data = new Date();
    //var dia = data.getDay();

    //document.write(dias_semana[dia]);

    //linhasOcultas = (dias_semana[dia]);

    //document.getElementById("#calendSemana").value(linhasOcultas);

    //var campoValor = document.getElementById("celular").value;
    //document.write(linhasOcultas);
    //document.getElementById('calendSemana').value += campoValor;

    //document.write("<input type='text' class='inputcad' id='calendSemana' name='calendSemana' maxlength='10' value='"+isso+"'>");

    //document.getElementById('#calendSemana').value = document.getElementById('#data_3').value;

    //document.getElementById('calendSemana').innerHTML = el.value;

}

<script type="text/javascript">
     function mostraDiv(valor) {
          if(valor == "Terça")
          {
                document.getElementById("TER").style.display = "block";
                document.getElementById("QUI").style.display = "none";
          }
          else if(valor == "Quinta")
          {
                document.getElementById("TER").style.display = "none";
                document.getElementById("QUI").style.display = "block";
          }
          else if(valor == "Selecione")
          {
                document.getElementById("TER").style.display = "none";
                document.getElementById("QUI").style.display = "none";
          }
}
</script>

<select name='dia' id='dia' class='inputselec' onchange='mostraDiv(this.value)'>
<option value="<? echo $_SESSION['dia_c']; ?>"><? if(empty($_SESSION['dia_c'])) { echo "Selecione"; } else { echo $_SESSION['dia_c']; } ?></option>
<option value='Terça'>Terça-feira</option>      
<option value='Quinta'>Quinta-feira</option>
</select>

<div id='TER' style='display:none'>
<select name='terca' id='terca' class='inputselec'>
<option value="<? echo $_SESSION['terca_c']; ?>"><? if(empty($_SESSION['terca_c'])) { echo "Horário"; } else { echo $_SESSION['terca_c']; } ?></option>
<option value='14:30'>14:30</option>
<option value='14:50'>14:50</option>
<option value='15:10'>15:10</option>
<option value='15:30'>15:30</option>
<option value='15:40'>15:40</option>
</select>
</div>
<div id='QUI' style='display:<? echo $mostraQ ?>'>
<select name='quinta' class='inputselec'>
<option value="<? echo $_SESSION['quinta_c']; ?>"><? if(empty($_SESSION['quinta_c'])) { echo "Horário"; } else { echo $_SESSION['quinta_c']; } ?></option>
<option value='09:40'>09:40</option>
<option value='10:10'>10:10</option>
<option value='10:30'>10:30</option>
<option value='10:50'>10:50</option>
<option value='11:10'>11:10</option>
</select>
</div>

Thank you very much to anyone who can help me ... Light and Peace!