According to Wikipedia:
"The municipality of Washington, D.C., celebrates April 16 as Emancipation Day. On that day in 1862, President Abraham Lincoln signed the Compensated Emancipation Act for the release of certain persons held to service or labor in the District of Columbia. The Act freed about 3,100 enslaved persons in the District of Columbia nine months before President Lincoln issued his famous Emancipation Proclamation. The District of Columbia Compensated Emancipation Act represents the only example of compensation by the federal government to former owners of emancipated slaves.
On January 4, 2005, Mayor Anthony Williams signed legislation making Emancipation Day an official public holiday in the District. Each year, a series of activities will be held during the public holiday including the traditional Emancipation Day parade celebrating the freedom of enslaved persons in the District of Columbia. The Emancipation Day celebration was held yearly from 1866 to 1901, and was resumed as a tradition and historic celebration in 2002 as a direct result of years of research, lobbying and leadership done by Ms. Loretta Carter-Hanes.
In 2007, the observance of this holiday in Washington, D.C. had the effect of nationally extending the 2006 income tax filing deadline from April 16 to April 17, a delay that will recur in April 2012. The 2007 date change was not discovered until after many forms went to print."