Rosh Hashanah Musaf-offering
Requires Beit HaMikdash
BaMidbar 29:1-2 The first day of the seventh month shall be a scared holiday to you when you shall not do any work of consequence. It shall be a day of sounding the ram's horn. You shall bring a olah-offering of a pleasing aroma to Hashem, [consisting of] one young bull, one ram, and seven yearling lambs, [all] without blemish.
We are commanded to offer an Musaf (Additional)-offering on the first day of Tishri, which is the Musaf-offering of the New Year.
The day of Rosh Hashanah (the New Year) being also the day of Rosh Chodesh (New Moon), two kinds of Musaf-offerings were to be brought. In the matter of precedence the general principle applied: "That which is offered more frequently has precedence." Hence the Musaf-offering of Rosh Hashanah preceded that of the Rosh Chodesh. If Rosh Chodesh fell on a Shabbat, three Musaf-offerings were brought, the Musaf-offering of Shabbat, the Musaf-offering of Rosh Chodesh, and the Musaf-offering of Rosh Chodesh. (Zev. 89a)