Mo. Rev. Stat. § 115.057: Offices may be maintained, to be open, when.

Each election authority shall maintain an office or offices sufficient for its purposes. The offices of each election authority shall be kept open during regular business hours on all election days and on all other weekdays, except legal holidays. The offices of each election authority shall also be kept open for four hours on the Saturday preceding each election and may be kept open at other times as determined by the election authority.