how to save several checkbox options in the same field of the bd

I need to know how to save more than one checkbox field option in a table field in the database. This is the way I form the chechkbox (they are the days of the week)

h4 Dia de la Semana
    label.checkbox-inline
        input#checkboxdo(type='checkbox',name='checkboxdia', value='0',class='dia')
        |  D
    label.checkbox-inline
        input#checkboxl(type='checkbox',name='checkboxdia', value='1',class='dia')
        |  L
    label.checkbox-inline
        input#checkboxma(type='checkbox',name='checkboxdia', value='2',class='dia')
        |  M
    label.checkbox-inline
        input#checkboxmi(type='checkbox',name='checkboxdia', value='3',class='dia')
        |  M
    label.checkbox-inline
        input#checkboxju(type='checkbox',name='checkboxdia', value='4',class='dia')
        |  J
    label.checkbox-inline
        input#checkboxvi(type='checkbox',name='checkboxdia', value='5',class='dia')
        |  V
    label.checkbox-inline
        input#checkboxsa(type='checkbox',name='checkboxdia', value='6',class='dia')
        |  S
.modal-footer
button.btn.btn-default(type='button', data-dismiss='modal',style='margin-left:0px;width:100px;') Cerrar
button#btnAgregarRecordatorio.btn.btn-primary(type='button',style='margin-left:10px;width:100px;',onclick="insertar_recordatorio()") Guardar

the way in which the values are collected is like this

   function insertar_recordatorio(){
    const usu = document.querySelectorAll('input[type=checkbox]:checked'); 
    if( $('#checkboxdo').prop('checked') ) {
        var dia=$('#checkboxdo').val();
    } if( $('#checkboxl').prop('checked') ) {
        var dia=$('#checkboxl').val();
    }if( $('#checkboxma').prop('checked') ) {
        var dia=$('#checkboxma').val();
    }if( $('#checkboxmi').prop('checked') ) {
    var dia=$('#checkboxmi').val();
    }if( $('#checkboxju').prop('checked') ) {
    var dia=$('#checkboxju').val();
    }if( $('#checkboxvi').prop('checked') ) {
        var dia=$('#checkboxvi').val();
    }if( $('#checkboxsa').prop('checked') ) {
    var dia=$('#checkboxsa').val();
    }             

    onAgregar(titulo,mensaje,hora,dia,CorreoExtra);
}

and that's how the insert is made

exports.insert = function(g, done) {  
var sqlstring  = 'INSERT INTO citas(pk,dia) VALUES(uuid_generate_v1mc(),$1) RETURNING pk';
var sqlvalues = [g.dia];
mpg.query(sqlstring, sqlvalues, function(err,result){
    mglobal.validarResultQueryInsert(err,result,nombre_modulo,sqlstring,sqlvalues, function(resvalid){
        if(resvalid.exito && result.rows.length > 0) 
          resvalid.pk =result.rows[0].pk;            
        done(resvalid);
    });      
});

}