Day was waived by Jacksonville on November 18 and fills the roster spot left vacant by the release of defensive lineman Tony McDaniel on November 13.
As Yates stated, Day reunites with 49ers defensive coordinator Robert Saleh, who was the linebackers coach for the Jaguars over the past three seasons.
Day was originally a fourth-round draft selection by the Jaguars in 2016. Over the past two years, he appeared in 22 games and registered 11 tackles, two sacks, and one pass defensed. This season, he appeared in six games and had three tackles, a sack, and one pass defensed.
Day played college football at Notre Dame, where he appeared in 48 games and started 32. During that time, he registered 141 tackles, 32 tackles for loss, eight passes defensed, 7.5 sacks, two forced fumbles, and two fumble recoveries. In 2015, during his senior year, Day finished with 45 tackles, 15.5 tackles for loss, four sacks, four passes defended, and two forced fumbles.