For centuries, Jewish custom has prohibited marriages at specific dates and times during the Jewish year. A strict interpretation of Jewish law prohibits work on certain days: Shabbat, Rosh HaShanah, Yom Kippur, and the first and last days of festivals, such as Passover and Sukkot. (See the Jewish holiday calendar). Since weddings historically involved a monetary transaction and the signing of a legal contract, both were considered forms of work. Thus, Rabbinic law prohibited weddings at those times.
Although many contemporary Jews and Jewish movements do not view weddings as legal business transactions, most rabbis maintain the custom of not officiating at weddings on these days. There is an additional reason not to officiate at weddings on Shabbat and festivals: the Rabbinic sages taught that weddings are not celebrated on these days “because we do not mix onewith another” (Mishneh Torah, Hilkot T’filah 1:2 based on Mishnah Mo-eid Katan 1:7).