New-York postmark is first-use listed as 1833 in American Stampless Cover Catalog.  Letter date inside is clearly 1824.  In addition to the information below, this item looks to be, by far, and earliest known use cover.
This 1824 letter from New York to Boston was written and signed by prominent New York lawyer and minister Horace Holden.  It is addressed to Samuel Armstrong, Boston printer and bookseller, thanking him for donating 100 pamphlets to Holden’s church in Manhattan.  A significant communication between two high ranking citizens.
Horace Holden was admitted to the New York Bar in 1811 and practiced law in New York City. During the war of 1812, he was stationed at Sandy Hook and was on General Colfax's personal staff. He ordained as one of the Ruling Elders of the Brick Presbyterian Church of Manhattan in March of 1823.
Samuel Turell Armstrong (April 29, 1784 – March 26, 1850) was a U.S. political figure. Born in 1784 in Dorchester, Massachusetts, he was a printer and bookseller in Boston, specializing in religious materials. Among his works were an early stereotype edition of Scott's Family Bible, which was very popular, and The Panoplist, a religious magazine devoted to missionary interests.
Armstrong began to withdraw from the printing business in 1825, and focused instead on politics. He was active in Boston politics during the 1820s, twice winning a seat in the Massachusetts General Court (state legislature). In 1833 he was elected the lieutenant governor of Massachusetts as a Whig, and served three consecutive annual terms. For most of the last term he was acting governor after Governor John Davis resigned to take a seat in the United States Senate. He lost a bid to be elected governor in his own right in 1836, but was elected Mayor of Boston, a post he held for one year.