Well there’s a long list of things, but for starters…
Lent
Clerical celibacy
Christmas - date of the birth of Christ
Missing mass mortal sin
…
Just to address one of the above, the discipline of celibacy stems from the nature of the priesthood itself, namely that a priest offers sacrifice (THE Sacrifice of the Mass). As we see already practiced in Old Testament times, priests rotated terms of service in the Temple and were required to practice sexual fasting during the time of their service. (We see this at the beginning of St. Luke’s gospel when he presents us with the account of Zachary and Elizabeth and the conception of John the Baptist).) So it is quite consistent with Scripture.
Following this example, but most especially the example of Christ, the High Priest of the New Covenant, when married men were chosen to be deacons, priests, or bishops in the early Church they were required to be continent or be disbarred from the clergy, following the Apostolic example, whom themselves were following the example of Christ in giving all to the furtherance of His Kingdom.
There were simple reasons for this: the Twelve were living celibate when they left their families to follow Our Lord and so were living with and like Him. Moreover, the Apostolic Offices of the Twelve was the highest office of the Church, which the Episcopal offices comes closest to but not equal to. The continence required of bishops followed the Twelve’s vocation to live totally for the Kingdom, as Our Lord says in Matthew 19 that some are eunuchs for the sake of the Kingdom.
At first this did not mean separation from one’s wife, but only separation from conjugal sex; this normally would not be a problem, since priests and bishops, and even deacons were chosen from the elders, i.e., those whose children were already raised.
The Church has always allowed couples who had no responsibilities towards offspring or others to mutually separate and enter religious life. In the first period of the Church when spirituality was higher than later (first 300 years) this was not so unusual and the Church following the practice of the Apostles as well as their example chose priests and deacons from among these spiritually mature elders, who though not separating from their wives who needed their financial and social support before such things as monasteries, lived like brothers and sisters with regard to sex. A great example of this in the West is St. Nicholas of Flue: after a prolific marriage, he and his wife spent the last 20 years in continence and he was blessed with prophecy and even living only on the Eucharist (15th century, I believe).
The whole history of this discipline of the Church is well-documented in Fr. Christian Cochini’s Apostolic Origins of Clerical Celibacy.
