The order of the Mishnah containing the tractate Rosh Hashanah is Moed.
Seder Moed, meaning “Order of Appointed Time,” is the second of the Mishnah’s six major divisions. It gathers laws concerning sacred times, especially Shabbat, festivals, fasts, pilgrimage observances, and the Temple-related calendar. Tractate Rosh Hashanah fits here because it treats the sanctification of months, the festival’s rituals, and its liturgy.
Within the traditional sequence of Moed, Rosh Hashanah is listed as the eighth tractate. Its four chapters move from the different New Year dates and the testimony of moon witnesses to rules for the shofar and the structure of the holiday prayers. The tractate therefore connects calendar administration with worship.
The main possible confusion is between an order and a tractate. Moed is the large division, while Rosh Hashanah is one work inside it. Other familiar Moed tractates include Shabbat, Pesachim, Yoma, Sukkah, Megillah, and Ta’anit. The other five Mishnah orders are Zeraim, Nashim, Nezikin, Kodashim, and Tohorot.