The Supreme Court’s new term begins on the first Monday of October, and typically ends at the end of June. 28 U.S.C. § 2 sets this date: “The Supreme Court shall hold at the seat of government a term of court commencing on the first Monday in October of each year and may hold such adjourned or special terms as may be necessary.”
28 U.S.C. §2 was passed by Congress in 1916, and the Supreme Court began its term on the first Monday in October in 1917.
The Judiciary Act of 1789 provided that the Supreme Court “shall hold annually at the seat of government two sessions, the one commencing the first Monday of February, and the other the first Monday of August.” The Court met for the first time on February 2, 1790.
In the mid-19th century, the Court began its term in December. During this time, the Court’s docket grew exponentially, and so Congress allowed the Court in 1866 to set its own start date; the Court moved that date to October. In 1873, Congress formalized this practice by passing a law that moved the Court’s term from the first Monday in December to the second Monday in October. It remained on that day until 1917.
The Court does sometimes hold special sessions for important cases. Examples include the 1942 case of Ex Parte Quirin and the 2003 case of McConnell v. Federal Election Commission.
*Source: David L. Hudson, Jr, The Handy Supreme Court Answer Book.