Christ was a Jew, so why don’t Catholics observe Judaism?
Catholicism is the completion of the Jewish religion. Had all of the Jews of Jesus’ time accepted him as the Messiah and entered the Church, there would have continued to be one true religion. But because Jesus of Nazareth was not accepted as the Messiah by the majority of the Jews of his time, a split occurred. Pre-Christian Judaism was the true religion; post-Christian Judaism is an incomplete religion because it lacks recognition of the Messiah it anticipates.