What Does Paul Mean by "That Day" in 2 Thessalonians 2:3?

From Tony Garland of Spirit and Truth:

There is considerable [y] confusion among Christians concerning passages which refer to a future "day" associated with the arrival of Christ. This confusion is mainly caused by assuming that all references to a future "day" involving Christ must either refer to His return for the Church at the Rapture or His final return in judgment at the Second Coming. However, Scripture reveals a third situation in which Christ is said to “come” which, unfortunately, many erroneously conflate with the Rapture...keep reading