If it be objected to us on this subject that we ourselves are accustomed to observe certain days, as for example the Lord's day, the Preparation, the Passover, or Pentecost, I have to answer, that to the perfect Christian, who is ever in his thoughts, words, and deeds serving his natural Lord, God the Word, all his days are the Lord's, and he is always keeping the Lord's day. He also who is unceasingly preparing himself for the true life, and abstaining from the pleasures of this life which lead astray so many—who is not indulging the lust of the flesh, but “keeping under his body, and bringing it into subjection,”— such a one is always keeping Preparation-day.
Again, he who considers that “Christ our Passover was sacrificed for us,” and that it is his duty to keep the feast by eating of the flesh of the Word, never ceases to keep the paschal feast; for the pascha means a “passover,” and he is ever striving in all his thoughts, words, and deeds, to pass over from the things of this life to God, and is hastening towards the city of God. And, finally, he who can truly say, “We are risen with Christ,” and “He has exalted us, and made us to sit with Him in heavenly places in Christ,” is always living in the season of Pentecost; and most of all, when going up to the upper chamber, like the apostles of Jesus, he gives himself to supplication and prayer, that he may become worthy of receiving “the mighty wind rushing from heaven,” which is powerful to destroy sin and its fruits among men, and worthy of having some share of the tongue of fire which God sends.
Source: Against Celsus (New Advent)