Let us see what Celsus further says of God, and how he urges us to the use of those things which are properly called idol offerings, or, still better, offerings to demons, although, in his ignorance of what true sanctity is, and what sacrifices are well-pleasing to God, he call them “holy sacrifices.” His words are, God is the God of all alike; He is good, He stands in need of nothing, and He is without jealousy. What, then, is there to hinder those who are most devoted to His service from taking part in public feasts.
I cannot see the connection which he fancies between God's being good, and independent, and free from jealousy, and His devoted servants taking part in public feasts. I confess, indeed, that from the fact that God is good, and without want of anything, and free from jealousy, it would follow as a consequence that we might take part in public feasts, if it were proved that the public feasts had nothing wrong in them, and were grounded upon true views of the character of God, so that they resulted naturally from a devout service of God.
If, however, the so-called public festivals can in no way be shown to accord with the service of God, but may on the contrary be proved to have been devised by men when occasion offered to commemorate some human events, or to set forth certain qualities of water or earth, or the fruits of the earth—in that case, it is clear that those who wish to offer an enlightened worship to the Divine Being will act according to sound reason, and not take part in the public feasts. For “to keep a feast,” as one of the wise men of Greece has well said, “is nothing else than to do one's duty;” and that man truly celebrates a feast who does his duty and prays always, offering up continually bloodless sacrifices in prayer to God.
That therefore seems to me a most noble saying of Paul, “You observe days, and months, and times, and years. I am afraid of you, lest I have bestowed upon you labour in vain.”
Source: Against Celsus (New Advent)