September 9th does not exist in one year of American history.  Which
year is it?  This is a trivia type question.

Hello srogers-ga

Thank-you for your question.

The year in question is 1752 and the day is missing due to the
authorities changing the calendar system in use (from the Julian to
Gregorian system).

"On September 2, 1752, an odd happening occurred that's still keeping
genealogists on their toes. On that day, the British Isles and all the
English colonies, including America, lost 11 days--September 3 through
13."
http://www.genealogytoday.com/columns/everyday/030902.html

"England and its American colonies did not adopt the reformed
Gregorian calendar until 1752. [...] Interestingly, Alaska did not
change from the Julian calendar to the New Style Gregorian calendar
until 1867 because, up to that point, it was part of Russia.  In order
to make the calendar adjustment, eleven days were dropped from the
month of September 1752."
http://www.ancestry.com/learn/library/article.aspx?article=3358

"The legal code of the United States does not specify an official
national calendar. Use of the Gregorian calendar in the United States
stems from an Act of Parliament of the United Kingdom in 1751, which
specified use of the Gregorian calendar in England and its colonies."
http://webexhibits.org/calendars/year-countries.html

"Putting the calendar ?right? involved a momentous change: ?losing?
several days from the existing calendar - by the 1750s, 11 days."
http://www.earlymodernweb.org.uk/emn/index.php/archives/2004/12/calendars-and-dates/

"In Great Britain (and to-become USA), the Gregorian calendar was
adopted much later - 11 days were dropped in September 1752."
http://www.timeanddate.com/date/leapyear.html

Calendar for the USA in 1752:
http://www.timeanddate.com/calendar/?year=1752&country=1

When a country changed from the Julian to Gregorian calendar:
http://www.everything2.com/index.pl?node_id=91530
http://www.rundetaarn.dk/engelsk/observatorium/print/gregorian.html

I hope this answers your query, should you require any further
assistance on this subject please do not hesitate to ask for
clarification.

Search Strategy:
://www.google.co.uk/search?q=calendar+1752
://www.google.co.uk/search?q=usa+calendar+1752
http://homepages.tesco.net/~jk.calisto/calisto/calendars/change_dates_jg.htm

Thank-you the answer was wright on.