The special election to replace former Rep. Jesse Jackson, Jr. (D-Ill.) will be held on April 9.
Illinois Gov. Pat Quinn (D) on Sunday signed a measure to set the date for the contest, the Associated Press first reported.
The Illinois legislature approved a bill on Wednesday allowing the election to be held outside the 115-day period. The governor and lawmakers defended the measure as a cost-saving move.
Jackson Jr. resigned from the House last month, citing his treatment for severe bipolar disorder. The Illinois lawmaker has been absent from Capitol Hill since June.
Jackson is also reportedly the focus of a Justice Department probe into allegations he misused campaign funds to redecorate a house.
Two former House members, Rep. Mel Reynolds and Rep. Debbie Halvorson, have announced that they will seek the Democratic nomination. They will be joined by state Sens. Toi Hutchinson, Napoleon Harris and Chicago Alderman Anthony Beale.
The primary will be held on Feb. 26.