In "History 1000 Years from Now" (written in 1901 but first published
in Mark Twain's Fables of Man, 1972) Twain wrote:

"Today no subject but the one -- the past -- can get much attention.
We began, a couple of years ago, with a quarrel as to whether the
dying century closed with the 31st of December 2899, or whether it
would close with the last day of last year, and it took the entire world
the best part of a year to settle it; then the past was taken hold of
with interest, and that interest has increased in strength and in
fascination ever since."

Everyone seems to be experiencing the same confusion this year.

Twain's own interest inspired quite a few writings in 1900-1902,
many of which can be found in Mark Twain's Fables of Man, The
Bible According to Mark Twain, and Letters from the Earth.

I have put "A Salutation Speech from the Nineteenth Century to the
Twentieth" online in several places, all of which probably have some
interest because of the other greetings included with them:

Anti-Imperialists Greet the New Century
http://www.boondocksnet.com/century/century_greet.html
Includes Twain's with greetings written for the same event by William
Jennings Bryan and Senator Richard F. Pettigrew.

Editors on Mark Twain's Anti-Imperialism
http://marktwain.about.com/library/texts/bl_ed_editors.htm
An editorial from the Springfield Republican discusses how Twain
was recruited to write the greeting.  The editorial from The Public
includes greetings written for the Red Cross Society by William
McKinley, Theodore Roosevelt and William Jennings Bryan as well
as Twain's.  I like to use that one as an example of Twain's
prominence at the time -- he was grouped with the president, the
vice-president-elect, and their opponent in the recently concluded
presidential election.

A Salutation Speech from the Nineteenth Century to the Twentieth
http://marktwain.about.com/library/texts/bl_salutation.htm
Includes a facsimile of the card version of Twain's greeting
published by Albert S. Parsons of the New England Anti-Imperialist
League that was based on an earlier draft of the salutation.

Jim Zwick