1. On September 18, 1805, the Lewis and Clark Expedition reached a major milestone in their journey by crossing the Continental Divide at Lemhi Pass in Montana. The pass, located in the Bitterroot Range of the Rocky Mountains, proved to be a major obstacle for the explorers, but its crossing enabled them to begin their descent towards the Pacific Ocean.

2. September 18, 1866 marked the opening of Montana's first public school in Virginia City. The school, which was initially built as a church, served as a central hub for education in the region and was instrumental in promoting literacy and knowledge throughout the territory.

3. On September 18, 1874, a group of Blackfoot Native Americans attacked a wagon train of settlers traveling through Montana's Marias River Valley. The attack, known as the Marias Massacre, resulted in the deaths of over 200 members of the Piegan tribe, including many women and children. The incident drew significant attention to the tensions between Native Americans and settlers in the region.

4. September 18, 1905 saw the opening of Montana's Bitterroot Valley Railway, which connected the towns of Hamilton and Darby via a 34-mile long railroad. The railway was significant in promoting economic growth in the region by enabling the transportation of goods, supplies, and people between the towns.

5. The Continental Divide Trail, a 3,100-mile long hiking trail that stretches from Mexico to Canada, passes through Montana and several other states. September 18 marks an important milestone for hikers on the trail, as it is typically considered the last day that favorable weather conditions exist before winter sets in. Many hikers aim to reach the Canadian border by this date to avoid the risk of being stranded by snow and cold temperatures in the mountains.