Matsya Purana says. ‘Those who observe a Vrata should not eat during Sanskaras (rites during a lifetime) and Pretashraadh (obsequial rites).’ Therefore a renounced person and widow, who observe the Vrata of Brahmcharya should not dine at such ceremonies even if they have been invited to do so. Sanskaras here should be understood as any of the rites such as birth, death, marriage etc.
Similarly Pretashraadh is the eleventh or twelfth day following a persons death, on which special rites are performed. Thus in such situations, or invitations for lunch, Brahmcharis and Sadhus should never dine.