As an example, let us take a doubt which might arise regarding the fast and abstinence on the vigil of a feast. (*This example is based on pre-1960 Church laws of fast and abstinence. —Publisher, 2000) The person knows that the vigils of certain great feasts are days of fast and abstinence from meat, but the question arises in his mind whether or not the day before the Feast of the Ascension is such a day. If he ate meat on that day, assuming that the day was not a day of fast and abstinence, but he had taken no pains to find out for certain, he would sin thereby, even though fast and abstinence were not actually prescribed by the Church. His duty is to make sure, if he can, whether or not it is a day of fast and abstinence, and to act accordingly. This he could ordinarily do by inquiry or by referring to a Catholic calendar, though circumstances might arise where it would be impossible at the time to resolve the doubt. In this latter case, he should refrain from eating meat.