Mont. Code § 61-10-122
The department of transportation or local authority may issue or withhold a special permit at its discretion or, if the permit is issued, limit the number of trips or establish seasonal or other time limitations within which the vehicle, combination of vehicles, load, object, or other thing described may be operated on the public highways indicated, or otherwise limit or prescribe conditions of operation of the vehicle, combination of vehicles, load, object, or other thing when necessary to assure against damage to the road foundation, surfaces, or structures or safety of traffic, and may require an undertaking or other security considered necessary to compensate for injury to a roadway or road structure. During harvest no permit may be denied to oversize harvest or harvest-related agricultural machinery solely on the grounds that the travel takes place on a Saturday or Sunday. No permit may be denied to dealers in implements of husbandry and self-propelled machinery solely on the grounds that the travel may take place on a Saturday or Sunday.
§ 61-10-122, MCA