December 13th 1636 is recognized as the birthday of the National Guard. On this date in 1636, the first militia regiments in North America were organized in Massachusetts. 

Based upon an order of the Massachusetts Bay Colony's General Court, the colony's militia was organized into three permanent regiments to better defend the colony. Today, the descendants of these first regiments, the 181st Infantry, the 182nd Infantry, the 101st Field Artillery, and the 101st Engineer Battalion of the Massachusetts Army National Guard, share the distinction of being the oldest units in the U.S. military. The Militia Act of May 8, 1792, permitted militia units organized before the May 8, 1792, to retain their "customary privileges." This provision of the militia act was perpetuated by the Militia Act of 1903, the National Defense Act of 1916, and by subsequent law.

December 13, 1636, thus marks the beginning of the organized militia, and the birth of the National Guard's oldest organized units is symbolic of the founding of all the state, territory, and District of Columbia militias that collectively make up today's National Guard.