Mo. Rev. Stat. § 115.329: Time for filing of petitions.

1. The secretary of state or any election authority shall not accept for filing any petition for the formation of a new party or for the nomination of an independent candidate which is submitted prior to 8:00 a.m. on the day immediately following the general election next preceding the general election for which the petition is submitted or which is submitted after 5:00 p.m. on the fifteenth Monday immediately preceding the general election for which the petition is submitted.

2. When a special election to fill a vacancy is called, no election authority shall accept for filing any petition for the formation of a new party or for the nomination of an independent candidate which is submitted after 5:00 p.m. on the day which is midway between the day the election is called and the election day.

3. When a special election to fill a vacancy is called to fill an unexpired term for state representative or state senator, the secretary of state shall not accept for filing any petition for the formation of a new party or for the nomination of an independent candidate which is submitted after 5:00 p.m. on the twenty-first day after the writ of election is issued by the governor pursuant to Article III, Section 14 of the Missouri Constitution, calculated by excluding the day the writ is issued.