Mo. Rev. Stat. § 128.040: Election of electors.

The qualified voters in each of said districts shall meet at their respective places of holding elections on the first Tuesday after the first Monday of November, in the year one thousand eight hundred and eighty, and on the first Tuesday after the first Monday of November every four years thereafter, unless the Congress of the United States shall appoint a different day, and in that case, on such day as the said congress shall appoint, and shall proceed to elect as many fit persons as the state shall then be entitled to elect, as electors of president and vice president, one of whom shall be a resident of each district.