Union soldiers filled the streets of Alexandria in 1862. Camps encircled the town. Regiments found places to shelter wherever they could. Hospitals operated in churches, schools, and private homes.
Given that backdrop, things were relatively quiet in December 1862--until, that is, a few days after the Battle of Fredericksburg (December 11 to 15).
In fact, they were probably quieter than usual, since the regiments had gone south under the command of General Ambrose Burnside. His ill-conceived plan was to get to Richmond via Fredericksburg and the Rappahannock River.
The Union casualties were triple the number of the Confederates, overwhelming the limited facilities in nearby Falmouth. Boats brought thousands of wounded to Alexandria and Washington.
On December 19, 1862, four days after the battle, Julia Wilbur wrote:
...on looking down Duke St. I saw ever so many soldiers coming. I soon found that these were the slightly wounded coming from the landing, 2 boats having arrived, & these were going to hospitals.
Then I looked over to the Bap. Ch. [Baptist Church, turned into a hospital]. Ambulances were standing there & they were carrying in the wounded. A corpse lay in the porch, right opposite the door, & 8 men have died in this hos. in 2 days. .... I went on to the Washington St. Hos. What numbers they were taking in here....A nurse asked me to go into her ward. There were but 2 men in it & these men were nearly well. I was there an hour & saw the beds nearly all filled up. One with an arm off, another with a leg off, one shattered with bullets & so on. —All day long the ambulances have been busy bringing them to the hospitals. Some were removed from F.[Fredericksburg] Saturday night & others Monday morning....
Mary Phinney von Olnhausen, the nurse at Mansion Street Hospital, also described the scene in her post-war memoir:
The whole street (Fairfax Street) was full of ambulances and the sick lay outside on the sidewalks from nine in the morning till five in the evening. Of course places were found for some; but already the house was full; so most had to be packed back again and taken off to Fairfax Seminary, two miles out. I have been so indignant all day. - not a thing done for them, not a wound dressed...They reached town last evening, lay in the cars all night without blankets or food, were chucked into ambulances, lay about here all day, and to-night were put back into ambulances and carted off again...
As for several other "Civil War celebrities":
The Battle of Fredericksburg brought Walt Whitman to Washington. He came in search of his brother, whom he incorrectly thought had died. His brother was fine, but Whitman stayed on as a friend and comfort to patients, while writing some of his most famous poetry.
Whitman found his brother at Lacy House, a home-turned-hospital in Falmouth. Clara Barton was also at Lacy House--she had come as the military prepared for battle and remained several weeks in the aftermath.
And, it was the Fredericksburg wounded that gave Louisa May Alcott her first and most vivid experiences as a nurse in a hospital in Georgetown. Untrained, she had three days' experience before "all was hurry and confusion" and she had responsibility for a ward of 40 patients.