No one knows when, exactly, Christ's return will occur. The phrase "day and hour" is specific enough to mean that the particular moment the event occurs is in view rather than a period of time within which it occurs; however, considerations about the suddenness of the event discussed below mean that the ultimate end cannot be what's in view either. We are told that angels do not know at which moment it will occur (I suppose that means the Devil doesn't either) and even the Son doesn't know. If only the Father knows, and the Son does not, that means there is no way for anyone to know--there is no way to figure it out and no revelation could be expected which would specify it.
Yet, the crowd which has tried to figure it out or reveal it outright continues to grow (including Wm. Miller of the 7th Day Adventists, Chas. Taze Russell of JW’s, Herbert W. Armstrong of the Worldwide Church of God, Edgar Whisenant, author of 88 Reasons, and Harold Camping of Family Radio, among others). Really, I don't know how much clearer Christ could have been on the subject. Those that pursue such a course have, minimally, fallen into error, and possibly, purposely, taken on the mantle of false prophet. May such folly cease to gain traction among the faithful!
Matthew (cf. Luke 17:22-37) tells us the time immediately prior to the return of Christ will be like the days of Noah before The Flood. Then, normal life (eating, drinking, marrying, farming, milling) proceeded right up to the moment sudden destruction came upon the world unaware. Despite Noah's preaching of righteousness and witnessing the construction of the ark, life prior to the flood was similar enough to what it had always been to lull his listeners into inattention. That, I believe, is the key point Christ was making--there was nothing about life as experienced by the masses prior to global judgment that signified that wrath was about to be poured out.
However, that point comes on the heels of Jesus elucidating very clear, noticeable, presaging signs that signified the end was near. How can these two points be compatible? They cannot be, if what Christ was referring to in this sudden ark-like deliverance from judgment at the end was to occur after the Abomination of Desolation (and all that goes with it). The only way the suddenness in the midst of regularity indicated by the description makes sense is if Jesus was not talking about the the ultimate end, but was referring to a period of judgment that started with that deliverance and finished with the ultimate end. This would be akin to the flood starting with Noah's family embarking on the ark, proceeding with a lengthy rain, and ending a year later with floodwaters receding and Noah's family disembarking.
In the example of Noah the judged were carried away by floodwaters, which was certainly passive for them, but was for God too, in the sense that it was indirect, through the agency of water. Noah, on the other hand, was personally, actively protected by God, who shut him in. There is a subtlety in the language of Matthew that could be seen to call upon the same dynamic. The word (paralambanetai) used to convey the action involved in taking the one in the field and at the mill has a range of meaning that allows it to be used for more personal, tender actions (like taking a bride) than either the word used of the floodwaters' action (eren) or for the ones left (aphietai) in the field or at the mill.
So, what is pictured is a cataclysm much worse than a mere 40 days of rain and a year of flood (i.e. 7 years of Great Tribulation) coming upon all on the earth at the end of the age. Some, like Noah in his day, will be actively removed from danger and taken by the hand of God (snatched or raptured), which fits quite well with Luke's "escape" (Koine: ekphrygien, to flee out from) and is pictured by the Parable of the Ten Virgins. Some will be left to their fate, carried off by the judgment overwhelming the whole earth as prophesied by Christ who will return at the end of it. The threshold the faithful need to cross is being ready for that Noachian escape popularly called The Rapture which comes suddenly, unknowably, but certainly in the course of everyday life.