Paschal Triduum
Except during the Paschal Triduum no limits are laid down for the time of celebrating Mass. The traditional rule included in the 1917 Code of Canon Law, forbidding (except to a limited extent on Christmas Night) celebration earlier than one hour before dawn or later than one hour after midday, was relaxed in the liturgical reforms of Pope Pius XII and completely abolished in those of the Second Vatican Council.