The folio "Sousa's Great Marches", compiled by Lester S. Levy, (C) 1975 by
Dover Publications, ISBN 0-486-23132-1, contains replicas of 23 original
sheet-music editions for the piano.  It's a fine book for the Sousa fan!
About the famous "Washington Post" march, Mr. Levy says:

"A march dedicated to a newspaper was a fanciful idea, and it came about
because of the newspaper's interest in encouraging literary expression in
the public schools.  In 1899 the Post staged a contest and offered prizes
for the best essays written by pupils, and it arranged to award the prizes
on the grounds of the Smithsonian Institution, where the participants and
visitors were to be entertained with musical selections by the United
States Marine Band, bandmaster John Philip Sousa conducting.

"Several days before the event, one of the proprietors of the Post asked
Sousa to write a march in celebration of the contest.  Sousa was delighted
to oblige.  'The Washington Post,' performed for the first time on June
15, 1889, before twenty thousand children and their parents and friends,
proved to be one of the half-dozen greatest that Sousa ever wrote."

The folio shows the cover-page art: the front page of the Washington Post
newspaper of the following day.  The first sentence gushes, "Never Before
Was There so Large a Gathering of the Scholars of the Public Schools
of Washington!"