Here's some thoughts from one of the more notable Church Fathers, John Chrysostom: “Some of our people fast with the Jews and keep their Sabbaths, and some even light lamps with them and take part in their festivals. I hate these festivals of the Jews. I hate their Sabbaths. I hate their new moons. I hate their sacrifices. God hates them, too. They were once venerable, but now they are contemptible. Once they were holy, but now they are profane. Do you see how great the difference is? Do you see how far they have fallen? When Christ was not yet come, their rites were acceptable; but now that the truth has come, the shadow is no longer useful. When the sun has risen, who needs the lamp? If someone admires the Jewish rites, he admires them as still valid. But if they are valid, then Christ has not yet come. And if Christ has not yet come, then He has not yet been crucified. And if He has not been crucified, then our sins have not been taken away. You see where this leads. Participation in their festivals is a denial of Christ." - John Chrysostom, "Against the Judaizing Christians," Homily 1 in Antioch, circa 367AD.