What is "mussaf"

Mussaf (also spelled Musaf) is an additional service that is recited on Shabbat, Yom Tov, Chol Hamoed, and Rosh Chodesh. The service, which is traditionally combined with the Shacharit in synagogues, is considered to be additional to the regular services of Shacharit, Mincha, and Maariv.

During the days of the Holy Temple, additional offerings were offered on these festive days. Mussaf is now recited in lieu of these offerings.

Mussaf refers to both the full service (which includes the Amidah and all Jewish prayer that follow that are normally recited during Shacharit) and the Amidah itself that is recited for Mussaf. The main addition is a fourth recitation of the Amidah specially for these days. It is permissible to recite the Mussaf prayer at any time during the day on these days. Nevertheless, the tradition is that it be recited immediately following Shacharit as a combined service.