 In 1907, when, in accordance with the rules in force since the time of Pope Pius V, feast days of any form of double, if impeded by "occurrence" (falling on the same day) with a feast day of higher class, were transferred to another day, this classification of feast days was of great practical importance for deciding which feast day to celebrate on any particular day.Pope Pius X simplified matters considerably in his 1911 reform of the Roman Breviary.In the case of occurrence the lower-ranking feast day could become a commemoration within the celebration of the higher-ranking one.