JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" name="dias[]" value="Segunda">Segunda &nbsp;
<input type="checkbox" name="dias[]" value="Terca">Terça &nbsp;
<input type="checkbox" name="dias[]" value="Quarta">Quarta &nbsp;
<input type="checkbox" name="dias[]" value="Quinta">Quinta &nbsp;
<input type="checkbox" name="dias[]" value="Sexta">Sexta &nbsp;
<input type="checkbox" name="dias[]" value="Sabado">Sabado &nbsp;
<input type="checkbox" name="dias[]" value="Domingo">Domingo &nbsp;
<input type="hidden" name="dias" value="">

JavaScript

var inputs = $('input[name="dias[]"]');
inputs.on('change', function () {
    var str = [];
    inputs.each(function () {
        if (this.checked) str.push(this.value);
    });
    $('input[name="dias"]').val(str.join(','));
    console.log($('input[name="dias"]').val());
});