[Answered ]-Django UpdateView using the validation of a custom create view

You simply need to tell your view to use your form.

class FestivoUpdateView(UpdateView):
    form_class = Formulario_nuevo_festivo
    ...etc...

Note there doesn’t seem to be anything in your form that makes it solely for new festivals, so you may want to rename it to something more generic (and PEP8-compliant), such as FormularioFestivo.