In this letter from April 8, Meade mentions one Gurowsky, whom he describes as “that crazy old man.” Adam Gurowski was indeed one of the eccentric characters who enlivened Washington during the Civil War. (You can read more about him here.) His emotional and sometimes almost unhinged behavior on the city streets even made President Lincoln eye him as a potential assassin. A Polish count, Gurowski had come to the United States in 1849 after a life of radical (and inconstant) political agitation. As the Civil War approached, the excitable Polish count allied himself with the Radical Republicans. Once the war began, he pestered Lincoln with numerous hectoring letters about war policy. “He was the perfect radical type, an uncompromising Puritan in the fold,” wrote Leroy H. Fischer in the article linked above. As such, George McClellan—whom Gurowski described as “that half ass half traitor”—and anything associated with him, including Meade, would have been anathema.
The fair that Meade mentions is the Great Central Sanitary Fair, held in June 1864 to raise money for the Sanitary Commission, which provided care for wounded soldiers. In his book Philadelphia and the Civil War, Anthony Waskie described the fair as “probably the greatest purely civic act of voluntary benevolence ever attempted in Philadelphia.” By the time it closed on June 28 the fair had raised $1,261,822.55.
The New York Tribune of yesterday informs the world it has been positively ascertained that I am and have been in constant correspondence with McClellan, and that this fact has destroyed all my chances for nomination as major general in the regular army, but it is not believed it will remove me from command. I know where this canard comes from. Grant told me that he had received several visits and innumerable letters from that old crazy man Gurowsky, all to the effect that I was completely under the influence of McClellan, and in constant correspondence with him, and urging Grant to relieve me. I saw Historicus’s last effort, and was greatly amused at the very powerful position that he assigned me in the despotism he asserts I have exercised in the face of the lieutenant general and others. I am sure I ought to be flattered that I am allowed to exercise such powers. It is a redeeming trait in the powers that be, and in my countrymen, that the base and persistent attacks on me have so signally failed, principally from the bad standing of my assailants. As to my being nominated for the regular army, I never dreamed of it, though I always believed the secret of some of the attacks on me was to remove a rival from some one who did want and expected to be nominated.

The Philadephia fair’s dining saloon, in one of the temporary buildings erected in Logan Square (Library of Congress).
I think it a pity Philadelphia was so late in getting up its fair. The subject will be so thoroughly exhausted that people will be tired with such matters. Still, there seems to be great spirit evinced by those who have it in charge.
I have now as a guest Lieutenant Colonel Strave, of the Russian Engineers, who seems a young man of intelligence. He came down with letters from Mr. Seward and Mr. Stanton.
Meade’s correspondence taken from The Life and Letters of George Gordon Meade, Major-General United States Army, Vol. 2, (New York: Charles Scribner’s Sons, 1913), pp. 188-9. Available via Google Books.