April 19 ...

You're about to be remembered forever if you are:
1775 - Major John Pitcairn
1943 - A resident of the jewish ghetto of Warsaw 
1993 - David Koresh or any other Branch Davidian,
1996 - Timothy McVeigh
2014 - The Tzarnaev brothers

Go ahead, look them up in your Funk and Wagnall's , uh, Wikipedia?

I more like to remember Major Pitcairn. His most famous quote, the exact words of which are not universally remembered the same, are:

Disperse, you rebels! Damn you! Lay down your arms and disperse!

This was followed by a command to open fire on Lexington Green. His account of the action, written a few days later, goes as:

Report on the Battles of Lexington and Concord

Major John Pitcairn

April 26, 1775

Boston Camp,
To: General Thomas Gage
Sir,
As you are anxious to know the particulars that happened near and at Lexington in the 19 th Inst agreeable to your desire, I will in as concise a manner as possible state the facts, for my time at present is so much employed, as to prevent a more particular narrative of the occurrences of that day.
Six companies of Light Infantry were detached by Lt Colo Smith to take possession of two bridges on the other side of Concord, near three in the Morning, when we were advanced within about two miles of Lexington, intelligence was received that about 500 men in arms were assembled, determined to oppose the Kings troops, and retard them in their march. On this intelligence, I mounted my horse, and galloped up to the six Light Companies. When I arrived at the head of the advance Company, two officers came and informed me, that a man of the rebels advanced from those that were assembled, had presented his musket and attempted to shoot them, but the piece flashed in the pan. On this I gave directions to the troops to move forward, but on no account to fire, or even attempt it without orders; when I arrived at the end of the Village, I observed drawn up upon a Green near 200 rebels; when I came within about 100 yards of them, they began to file off towards some stone walls on our right flank. The Light Infantry, observing this, ran after them. I instantly called to the soldiers not to fire, but surround and disarm them, and after several repetitions of those positive orders to the men, not to fire, etc. some of the rebels who had jumped over the wall fired four or five shots at the soldiers, which wounded a man of the Tenth and my horse was wounded in two places, from some quarter or other, and at the same time several shots were fired from a meeting house on our left. Upon this, without any order or regularity, the Light Infantry began a scattered fire, and continued in that situation for some little time, contrary to the repeated orders both of me and the officers that were present. It will be needless to mention what happened after, as I suppose Colo Smith hath given a particular account of it..
I am, Sir, Your Most Obedt
Humble Servant
John Pitcairn

The Pitcairn Report 

What else transpired that day has become a mixture of fact, fable and myth. The fact is that "the shot heard round the world" happened on April 19, 1775. That is why the 19th of April is called Patriots' Day and is a holiday in many places, notably Massachusetts and particularly Boston. That's also the reason that McVeigh and Tsarnaev chose to do what they did on the days they did it - McVeigh as a response in date to the Branch Davidian invasion and slaughter by Attorney General Reno (my opinion - your opinion and your mileage may vary according to your own individual driving habits) and the Tsarnaevs because it was a convenient day to blow up as many people in one place in Boston as would be possible with their pressure cookers.

Ralph Waldo Emerson chose to immortalize the events with his famous poem:

"Hymn: Sung at the Completion of the Concord Monument, April 19, 1836" 

By the rude bridge that arched the flood,
Their flag to April’s breeze unfurled,
Here once the embattled farmers stood,
And fired the shot heard round the world.

The foe long since in silence slept;
Alike the conqueror silent sleeps;
And Time the ruined bridge has swept
Down the dark stream which seaward creeps.

On this green bank, by this soft stream,
We set to-day a votive stone;
That memory may their deed redeem,
When, like our sires, our sons are gone.

Spirit, that made those heroes dare,
To die, and leave their children free,
Bid Time and Nature gently spare
The shaft we raise to them and thee.

 Emerson's son, Edward Waldo Emerson,who edited The Complete Works of Ralph Waldo Emerson (1904), noted: 'In the early editions of the Poems the date is given as 1836. This is a mistake. The Middlesex Yeoman gives the account of this celebration in 1837, and on the original slip in my possesion some one sending it to a friend at that time, has written "Sung by the people on battle-ground at the completion of the monument, 4th of July, 1837."'

Emerson, either one actually, was not there at Concord Bridge. Neither is the bridge that you see today at Concord, as it is a reconstruction of what was originally there and may or may not resemble the original (being a former bridge designer at one point in my life, I can fairly confidently say that you would never build it the same the second time). Nonetheless, several thousand painters who were there, have been there, or have never been there have succeeded in painting not only the bridge but also the various interpretations of battle scenes that there took place.

 [source]

Overall, the initial clash at Lexington Green was somewhat muddled. [1], [2], [3], and reported in an eyewitness account written nearly 50 years later by a no longer young man who was there [4].

But, it will always be known as the shot heard 'round the world.*
 Incidentally - during the British retreat, the Minutemen kicked butt according to all accounts. Pitcairn lost his mount, having it shot "out from under him" in word if not in total deed and had to walk back to Boston.

*Unlike NASA launching a space shuttle containing 4 cattle orbiting the earth 3 times before landing, thus becoming the herd shot 'round the world.