A Bajoran military officer, Day was secretly a member of the Alliance for Global Unity, or "The Circle."

In 2370, Day participated in the takeover of Deep Space Nine. He served under the titular command of General Krim, but actually followed orders from Minister Jaro. Day tried to prevent Starfleet personnel from informing Krim that the Circle was receiving covert support from the Cardassians.

When Krim did learn of the Circle's Cardassian backing, Day resisted Krim's order to return the station to Starfleet control. In the ensuing fray, Day tried to kill Commander Benjamin Sisko, but instead killed Bajoran resistance hero, Li Nalas, when Nalas took the phaser blast meant for Sisko.