Here is why June 14 is Flag Day
June 14 is the US Army's birthday. According to this article available online from the US Army's Chief of Military History:

On 14 June 1775 the Second Continental Congress, meeting in Philadelphia, adopted a resolution under which ten companies of expert riflemen would be immediately raised, six in Pennsylvania, two in Maryland, and two in Virginia. The "compleated" companies were to "march and join the army near Boston, to be employed . . . under the command of the chief Officer of that army." On the following day the Congress elected George Washington, Esq., of Virginia to be general and commander in chief "of the forces raised and to be raised in defence of American Liberty."