As it was in the days of Noah, so it will be in the days of the Son of Man; they were eating and drinking, marrying and giving in marriage up to the day that Noah entered the ark, and the flood came and destroyed them all. Similarly, as it was in the days of Lot: they were eating, drinking, buying, selling, planting, building; on the day when Lot left Sodom, fire and brimstone rained from the sky to destroy them all. So it will be on the day the Son of Man is revealed. On that day, a person who is on the housetop and whose belongings are in the house must not go down to get them, and likewise a person in the field must not return to what was left behind. Remember the wife of Lot. Whoever seeks to preserve his life will lose it, but whoever loses it will save it. I tell you, on that night there will be two people in one bed; one will be taken, the other left. And there will be two women grinding meal together; one will be taken, the other left." They said to him in reply, "Where, Lord?" He said to them, "Where the body is, there also the vultures will gather." (Lk17:26-37)
Natural calamities, like flood in the days of Noah and the fire and brimstone in the days of Lot, will happen. In those days the people were not ready for it. They were worried about their daily material and social happenings. They were not prepared and they were not aware of the events that happen.
But here in Jesus days he speaks about political happenings. The Roman army and its arrival into the land of Israel is the context on which Jesus uses his arguments. (The vultures here refer to the Romans). This makes sense of the warnings. When the legions arrive, the best thing to do is to get out and run.
Day of the son of man is similar to these. The day will come unexpected. People will be worried about eating and drinking etc. Destruction is due for the people who are not prepared for the day of the son of man. Jesus warns awful destruction upon people for the failure to heed his message.
