When speaking of Saints shared by both Western and Eastern churches (those prior to the 1054 schism) tomorrow’s feast, Sts. Constantine and Helen, comes to mind. Constantine’s move to the ancient city of Byzantium in 324, which he solemnly inaugurated in 330 and named it after himself, thus transferring the imperial rule from Rome to this new location, also gave it a new title – New Rome (here). And while St. Helen is seen and venerated among Catholics, St. Constantine is a different story. I’m really sure what it is that they have on St. Constantine, whether it’s a question of him starting a new Rome or the fact they consider him rescuing the Christian church as “fantasy” (here).
One thing I always think about when I think of St. Constantine is something which might not particularly be considered as being one of his major contributions to Christianity. I think of some of the laws he enforced. Namely, he declared Sunday to be the official day of rest. Markets were banned and public offices were closed (there were no restrictions, however, on farming work which was the work of the great majority of the people). (here)
Ironically enough, whether Catholics commemorate only St. Helen and not her son and Protestants commemorate neither one nor the other, it is the sanctity of the day of Sunday which all Christians share.