Stephen Zachary Day (born June 15, 1978) is an American former right-handed sinker-ball pitcher in Major League Baseball. He played for two teams from 2002 to 2006.

Day graduated from La Salle High School in Cincinnati in 1996, where he played basketball, baseball and golf and was prep player of the year in Cincinnati in 1995. He went on to attend the University of Cincinnati, studying pre-business. Day was drafted in the fifth round in 1996 by the New York Yankees. He spent four seasons in the organization before being sent to the Cleveland Indians alongside Jake Westbrook in July 2000, to complete a trade for David Justice. In July 2001, Cleveland dealt Day to the Montreal Expos for outfielder Milton Bradley.

Day made his major league debut for the Expos on June 15, 2002, his 24th birthday. He got the win, and was the first pitcher to debut and win on his birthday since at least 1901, possibly ever. He finished 2002 at 4–1 with a 3.62 ERA in 19 games. He also had one save, in which he pitched the final three innings in perfect fashion (0 runs, 0 hits, 5 strikeouts) to nail down a 4–3 win against the Florida Marlins.

Day made 23 starts, going 9–8. He led the majors in wild pitches with 13. He was initially named NL Rookie of the Month in April, but the award was revoked and given to Hee-seop Choi due to Day having reached the rookie limit of service days in 2002.

On May 12, Day hit his first and only career home run, off Wes Obermueller. Day's 2004 season was cut short due to injury. He hit the Disabled List in July with a shoulder injury and then again in August after breaking his middle finger trying to bunt. He made 19 starts, having a respectable 3.93 ERA despite going 5-10 for the Montreal Expos, who relocated and became the Washington Nationals after the season.

Day pitched in 12 games for the Nationals before being traded to Colorado.

He was 0–1 in 5 games for the Rockies.

Day began the season with the Rockies but was designated for assignment after posting an ERA of 10.80 in 3 starts.