If the din of nedava is talui in the fact you cannot bring a nedava on shabbos why should you be mechalek before or after shkiah? AS everyone already said the dinim of tefilah are all talui in your kabalas shabbos. Therefore there is no hechrech and reason to be medayeik the lashon of the MB. I will bli neder try to be meayin more in the seforim.
Menu