There is a tradition that amongst the ways the Greeks attacked Judaism was to seek to eradicate Kiddush HaChodesh, the determination of the new moon. Looked at in isolation it seems a strange mitzvah to oppose. To target Rosh HaShanah, Yom Kippur or Pesach, to attempt to abolish Shabbat and kashrut - this might have been the more obvious way to go. But Kiddush HaChodesh? Surely one of the least essential Jewish practices!
The Greeks knew what they were doing. It always seems to happen with antisemites that the aspect of Judaism they ridicule and attack is unfortunately well chosen. Kiddush HaChodesh is an example. It is a practice on which hangs the whole of the Jewish calendar and hence the whole pattern of Jewish observance. Jews who lose touch with the calendar lose their Judaism. A life without Jewish events loses Jewish identity.
Hirsch put it positively: “The catechism of the Jew,” he said, “consists of his calendar”. The dates a Jew holds sacred show you what that Jew believes in. Rosh HaShanah means the belief in creation and a Creator. Yom Kippur means personal responsibility for one’s deeds. Sukkot is Divine protection. Pesach is freedom for every human being. Shavu’ot is moral law. Without the calendar there is no Judaism.