1. 1775: Battles of Lexington and Concord - On April 20, 1775, the one of the most significant events in the American Revolution took place in Massachusetts - the Battles of Lexington and Concord. It was the first military encounter between British troops and American colonists that ignited the flame of a revolutionary war that lasted for eight years.

2. 1836: Massachusetts abolishes debtors' prisons - On April 20, 1836, Massachusetts became the first state in the U.S to abolish debtors' prisons. It was a progressive move that abolished imprisonment for non-payment of debts and reflected the notion that people should not be imprisoned solely for being poor.

3. 1861: Governor Andrew raises troops - On April 20, 1861, Governor John Albion Andrew of Massachusetts signed a bill to raise troops to defend the Union in response to President Lincoln's call for volunteers. He was one of the first governors to respond to the plea and raised fifteen regiments comprising of 14,000 men.

4. 1898: Boston Marathon held for the first time - On April 20, 1898, the Boston Marathon was held for the first time, making it the oldest continuously running marathon in the world. The race was initiated by the Boston Athletic Association as a way to promote the sport of running and has since become a beloved tradition.

5. 2013: Boston Marathon Bombing - On April 20, 2013, two homemade bombs exploded near the finish line of the Boston Marathon, killing three people and injuring more than 260 others. The tragic event shook the entire state of Massachusetts and the country, leading to one of the largest manhunts in U.S history for the perpetrators. The attack also led to renewed unity and solidarity among Bostonians and Americans, with people coming together to support the victims and law enforcement.