1. Maryland becomes a state: On December 15, 1788, Maryland officially became the 7th state to join the United States. This came after a long period of debate and negotiation, with some prominent Marylanders initially opposing the idea of joining the federal union.

2. Death of Johns Hopkins: Johns Hopkins, the famous businessman and philanthropist who founded Johns Hopkins University and Hospital, died on December 15, 1873. His legacy has had a major impact on Maryland and on the field of medicine as a whole.

3. Battle of Antietam: One of the bloodiest battles of the Civil War, the Battle of Antietam took place on September 17, 1862. However, the aftermath of the battle saw Union General Ambrose Burnside relieved of command on December 15, due to his perceived failures during the campaign.

4. The Great Baltimore Fire: On February 7, 1904, a massive fire swept through downtown Baltimore, destroying over 1,500 buildings and causing millions of dollars in damage. The fire began on a cold December 15 night in a dry goods warehouse and quickly spread, leading to one of the most significant events in Baltimore's history.

5. First-ever commuter train: On December 15, 1830, the first-ever commuter train left Baltimore and traveled to Ellicott City along the Baltimore and Ohio Railroad. This was a major step forward in transportation and helped to kickstart the development of commuter towns and suburbs throughout Maryland and beyond.