הלכה על במדבר 29:33

The commandment of the additional sacrifice on the eighth day of Sukkot, which is called Shemini Atseret: To sacrifice the additional sacrifice on the eighth day of the holiday of Sukkot - and that is the additional [sacrifice] of Shemini Atseret - as it is stated (Leviticus 23:27), "and you shall sacrifice a fire-offering to the Lord; it is an atseret (convocation), etc." And in the Order of Pinchas (Numbers 29:36-38), it is explained at length. And in the explanation, they, may their memory be blessed, said (Yoma 2b) that it is a festival on its own. And therefore, we should count this additional sacrifice as a commandment on its own.