Date: December 19, 1777 - June 19, 1778
This was the site of the winter encampment for the Continental Army in 1777-78.
Valley Forge wasn't Washington's first choice of location. My first choice would have been the Bahamas, but apparently that's quite inconveniently located. The valley was mostly uninhabited, and he felt it that would be easier to house and feed an army in a larger town. Congress, on the other hand, wanted the army nearby - keeping it between them (in York) and the British army (in Philadelphia). Valley Forge was close enough to Philadelphia to harass British troops there, but far enough away to prevent a surprise attack from the British. A life without surprises. What a thing to wish for.
Washington did have a point, though - the army had trouble getting supplies, and Valley Forge had no housing for the troops. And even though that winter was mild, it was cold for an army where a third of soldiers didn't have any shoes. Mild temperatures also meant it was damp, so disease ran rampant - thousands died.
All of which makes Valley Forge sound like a complete disaster, but the astonishing thing is - it wasn't. The better-equipped soldiers took the patrols; the others built houses. Nathanael Greene was named quartermaster and found supplies. They invented subsistence rations for themselves when food ran low. Baron Von Steuben and others arrived and began to drill the troops in military tactics and in basic camp layout and sanitation - helping stop the spead of the disease problem.
Far from being the catastrophe it seemed, Valley Forge is legendary today as the 'proving ground' of the American army. I imagine it would be of little comfort to those who suffered there. "Yes, you caught a terrible disease - but well done, you really proved yourself!"
The army emerged as a capable fighting force, though - winning a victory almost immediately at the Battle of Monmouth.