Big Switch There is definitely a switch of topics from the soon coming Destruction of Jerusalem and a future Coming of Jesus at the End of the world. There is an emphatic change by the words Jesus used here: "Verily, I say unto you" and the conjunction "but." (Gk. peri de, 'but concerning...') The first phrase put a finish to the topic of the End of the Mosaic Age, and the second introduced the topic of the Second Coming which would put an end to the Gospel Age.
The two sections are marked differently, not only by the use of "days" versus "day," but also the listing of many "signs" versus an unannounced Coming with "no signs."
Misconceptions Jesus seemed to be trying to clear up the misconceptions of the disciples (as well as rabbis) who would equate the horrible idea of the Destruction of the Temple with the End of the World! This was the same mistake the Jews made at the first Temple destruction in 586 B.C. "The Temple, The Temple" was their security assumed back then. God would not let it be destroyed, so they thought, for that would be like the end of the world (See Isaiah's warnings.)
For illustration, when the Disciples asked about "the End", they used the Greek word, sunteleias, which is used of the End of the World, or "The Harvest" as in Matthew 13:39. But when Jesus mentioned, "the end" in this first section of the Olivet discourse, He always used telos instead. This designated just the End of an Age. (In this case the End of the Mosaic Age.)
Signs Then, No Signs Later In the first section of Matthew 24, there were a "variety" of signs mentioned; signs that would lead the disciples all the way to the Destruction of Judea. (Notice hoe many times Jesus said "you" to the disciples!) But, in the second section, when Jesus ended His several illustrations (v.36ff-25:1ff) He always made the "same" conclusion: "You just don't know!" No signs!
This separation of topics can be seen as the best interpretation if Chapter 23 and 25 are included in the research. Jesus set the mood for the Destruction in 23, and highlighted the Consummation in chapter 25.
Sun, Moon, Stars Note that much of the imagery used in the first section (which to modern minds seems extraordinary) was familiar to the literate rabbis as common figures of speech used by their ancient prophets. Those metaphors were used of the downfall of nations. (Just as the nation of Israel was to collapse.) The change to ordinary, everyday occupational conduct in the second section more than hints of a change in topics.
Those Days, That Day Thus the switch in verbal usage from those days to that Day, as signifying a switch in topics, is underscored by several other "switchings" in this Olivet Discourse.
THOSE DAYS 24:1-34----->transition 24:35-36------>THAT DAY 24:36-25:46