In 2370, Day participated in the attempt to take over Deep Space 9 under the command of General Krim, with whom Day constantly argued as to what course of action to take in the mission. Although the space station at first appeared deserted, Day's infiltration team ultimately learned that a small team of Starfleet officers headed by station commander Benjamin Sisko remained behind to prevent the takeover.
While searching for Sisko, Day was lured into and trapped within a holosuite by the Starfleet commander. After being fooled by holographic representations of Starfleet personnel, along with Odo and Li, Day was informed by Sisko that The Circle was receiving support from the Cardassians. Day refused to believe Sisko, however, and, after being released from the holosuite, continued the search for Sisko while neglecting to pass on Sisko's information to General Krim.
After finally learning the truth about the Cardassians giving aid to the Circle and that Day had failed to relay Sisko's message, General Krim opted to return command of the station to Starfleet. Angered by this turn of events, Day attempted to kill Sisko with a phaser, but accidentally killed Li Nalas, who threw himself into the blast to protect Sisko. (DS9: "The Siege")
Colonel Day was played by Steven Weber.
Day's given name was not mentioned on-screen but is established in the script for "The Siege", which also describes him as "a rugged, cold-eyed man about forty [...] far more political [than General Krim] and is Jaro's eyes and ears in this operation, more like an SS attache. There is always a subtle tension between these two. Day thinks Krim is arrogant but knows he needs the support of the military." (Star Trek: Deep Space Nine Companion - A Series Guide and Script Library)
Day appears in the Prophecy and Change short story "Ha'mara", which states he was a member of the Bajoran Resistance during the Occupation of Bajor and was rumored to belong to one of the more extremist cells. Later, in 2369, he is in Ashalla when Benjamin Sisko and Kira Nerys become trapped in the "Paths of the Lost" after an explosion at the Taluno Library. Although he leads the rescue effort, Day secretly hopes that if Sisko dies, the Federation will withdraw from Bajor.
Day is also mentioned in the Worlds of Star Trek: Deep Space Nine novella "Bajor: Fragments and Omens", in which First Minister Asarem Wadeen tells Krim that Colonel Day's actions amounted to betrayal.