When is the Lord’s Return?
Was “no man knows the day or the hour” (Mt. 24:36) a reference to Rosh Hashanah?
Answer: No.
While there is a certain sense to the idea that God would end the world at the end of some specific week -- Saturday night, the end of 7-days, in reality, it makes no difference when He does this at all, as I will (try to) explain.
You see, there are dozens of passages that, if we think carefully, relate to us when/how the Lord will return, and they have nothing to do with any specific date on earth -- including Rosh Hashanah. Here are three such examples:
1 Thessalonians 4: “Then we who are alive and remain will be caught up together with them in the clouds to meet the Lord in the air, and so we shall always be with the Lord.”
1 Thessalonians 5:2: “For you yourselves know perfectly that the day of the Lord so comes as a thief in the night.”
Luke 12:39: “But know this, that if the master of the house had known what hour the thief would come, he would have watched and not allowed his house to be broken into.”
The day of Christ's return represents the eventuality of all the faithful dating back to the very beginning. God does not play favorites; it would be egregiously unfair that a single generation would be the sole recipient to witness the greatest event in the history of the world.
Suppose the last day of history is December 31, 3000. Since we will physically die before then, how can we witness Christ’s return? Why are we told to be prepared for an event that has apparently never occurred to countless multitudes that have already died in Christ (Jas. 5:7-8, Heb. 10:25, 1 Cor. 1:7, 2 Pet. 3:9, Matt. 24:42, Phi. 3:20, etc.)?
Then, there is this: How then does Christ “return” to those with whom He is already present (2 Cor. 5:8)? Are we really to somehow exist as bodiless, semi-triumphant ghosts awaiting the Lord's return? Would such an intangible existence not parallel the dreadful circumstances of demons, unclean spirits forever in search of a body (Matt. 12:43, Lk. 11:24)?
Further, was Stephen not in the Presence of God, just as he tells us that he was in Acts 7:56? Would the thief on the cross not really be in paradise with Christ, just as Jesus promised him that he would (Luke 23:43)? Does Christ never return as a “thief in the night” to any of the vast numbers of obedient saints that have departed this world, fully assured that they will see Him return?
If not, why then does Scripture remind us over 300 times about the certainty of the event? No one seems to pause and reflect on these things. In the Book of Revelation, it is written:
Revelation 1:7: “Behold, He is coming with the clouds, and every eye will see Him, even those who pierced Him; and all the tribes of the earth will mourn over Him”
This verse speaks to all the tribes of the earth. Surely we are not to believe these words apply only to judgment on Jerusalem or Rome in 70 and 78 A.D. (respectively)? These are all questions that demand an answer for those seeking the truth.
Our intractable problem is that we feel we will be “waiting” in heaven. That is, everyone believes there must be some amount of time between our departure from this life and our eventual encounter God. But that is illogical. You cannot speak of "time" in eternity, because that is just as nonsensical as believing that God had a beginning. When we traverse the threshold of time into eternity at death, time as we experience it ceases to exist entirely.
Let me try this: If there was a time “before” something in eternity, then there must, of necessity, have been a time even before that. This represents an infinite regression of "yesterdays," but there aren't any. There is only today in heaven just as God has told us: "I am." (Ex. 3, John 8, etc.) If we ignore this distinction we are rejecting dozens of passages in Scripture. Paul tells us something we should remember:
1 Corinthians 14:33: “[God] is not a God of confusion…”
It should be clear that God has not conveyed logical impossibilities to us. There are at least two sources that relate to us how God will return. And both of them suggest that He will not return to Earth (per se). From the Book of Acts:
Acts 1:11: “[Two angels] also said, ‘Men of Galilee, why do you stand looking into the sky? This Jesus, who has been taken up from you into heaven, will come in just the same way as you have watched Him go into heaven.’”
Very similar circumstances are echoed in the Letter to the Thessalonians. Upon the “last trumpet” (1 Cor. 15:52), we step out of time into God’s immediate Presence; there is no waiting for anything in heaven:
1 Thessalonians 4:17: “Then we who are alive and remain will be caught up together with them in the clouds to meet the Lord in the air, and so we shall always be with the Lord.”
Neither of these passages are relating to Rosh Hashanah. Note too, that these "clouds" are not on earth: they are the "clouds of heaven." Interestingly, Paul speaks of "we who are [still] alive." We should understand that Paul is trying to assuage the fear that his audience's [Thessalonians] lost loved ones would never see Christ. Then he states "then, we who are [still] alive" will be caught up with them. Are we too not "[those] who are alive?" Yes, we will be caught up to paradise just as Paul told the worried Thessalonian saints.
Clearly, 1 Thessalonians 4:17 parallels Acts 1:11, where Christ is to return with the clouds of heaven. And, when He returns, we are told that He will destroy the material universe with intense heat (2 Pet. 3:10). There is no way around this.
I've included a figure below intended to depict how every faithful soul is immediately ushered into the Presence of the Christ (dotted line). Note that time ceases to exist for us as we do this. Few who have died did so on Rosh Hashanah (although some may have):

Traversing the threshold of Time into eternity is analogous to the effects of Special Relativity: E = mc2.
Consider an analogy: Suppose we see our close relatives off on a flight to another country. But, as soon as they arrive, they are thrilled to discover that we are on the plane with them. The reason that no one is yet in heaven (Acts 2:34) is that we are not yet there either. But as soon as we arrive, all of the faithful dating to the end of physical time will be there along with us in that same eternal moment.
Christ could return in five minutes, since no one can know when the event will occur per Matthew 24:36 in the OP. Based entirely on Scripture, when Christ returns is completely irrelevant: we will all witness the event indistinguishably from one another.
Think about this. What event in everyone's life comes to them suddenly without warning -- as a thief in the night? We might encounter some fatal auto accident in a flash, without any warning. Or, we may be bedridden, expecting everything to be at it was so many days before, all without incident.
Then, in a moment, we each experience our last day: the end of our physical life on earth. This is the day we step into eternity, a realm where time ceases to elapse (certainly, as we now recognize it). Christ will never set foot on Earth again: We will step into His Presence in the clouds (Acts 1:11, 1 Thess. 4:17).
I have posted another partial answer to this elsewhere. Providing a complete answer to your question is very difficult since I can only scratch the surface of this subject here. However, NO, Christ will not return on to us on Rosh Hashanah -- unless it by sheer coincidence that our death happens to coincide with that day. Our death and the Lord's return are essentially synonymous -- a very difficult concept to relate to others.