No-one except God the Father, not even the angels or Jesus himself,  knows when the last days or the end of the world, or the return of Christ will happen.  This knowledge must have been one of the 'privileges' of divinity that God the Son chose not to bring with him into his human form - so that he could be truly human and participate in our human frailty.

This is a warning to us not to waste our time and energies trying to work it out.  We must trust God that he knows when the right time will be. We are to try to be ready for it all the time.  If we have accepted Christ and are doing our best to lead obedient lives, we don't need to know when all those things will happen - we'll be ready for him any time.

Quest Study Bible note, p 1404.  When Christ became human, he gave up the right to use his divine abilities for his own purposes (see Philippians 2: 6-8).  Because Jesus submitted to the Father in everything, he left the matter of the timing of his return toi the Father, too.  People today who claim to know the day of Christ's return are claiming to know more than Jesus did, and are false prophets.

New Bible Commentary, p 937.  V 36 is remarkable not only as the only admission of ignorance by Jesus, but also, paradoxically, because it at the same time places him above the angels and second only to the Father.  This view of the status of the Son is equalled in this gospel only in chapter 11: 27 and 28: 19.

Vs 37 - 39
'As things were in Noah's days, so will they be when the Son of Man comes. In the days before the flood they ate and drank and married, until the day that Noah went into the Ark, and they knew nothing until the flood came and swept them all away.  That is how it will be when the Son of Man comes.
NLT.  "When the Son of Man returns, it will be like in Noah's day.  In those days before teh Flood, teh people were enjoying banquets and parties and weddings right up to the time Noah entered his boat.  People didn't realise what was going to happen until the flood came and swept them all away.  That is the way it will be when the Son of Man comes.

Jesus compares this time - the time of the coming of the Son of Man - to the time of Noah.  Except for Noah, people just continued on their own way, living their lives as they pleased, ignoring God.  when the flood came they were totally unprepared for it.  God had warned Noah,  but no-one else would listen.  Jesus warns that the same thing will happen again if we don't listen now.

Vs 40 - 42
Then there will be two men in the field; one will be taken, the other left; two women grinding at the mill; one will be taken, the other left.  Keep awake, then; for you do not know on what day your Lord is to come.

When Jesus comes, those who are ready will remain with him (or be taken to heaven with him) and the others will miss out.  We need to be prepared at all times because there will be no warning - there will be no 'evacuation drill'!

Vs 43, 44.
Remember, if the householder had known what time of night the burglar was coming, he would have kept awake and not have let his house be broken into.  Hold yourselves ready, therefore, because the Son of Man will come at the time you least expect him.'

Another illustration.  Jesus will return like a thief - with no prior notice.  He's not going to send us an email saying, 'Ill be arriving on the 11.30 bus on Thurday week'! He said he'd come like a burglar.  Burglars don't let us know when they're coming, so we need to keep thr house secure at all times.  Burglar aslo take things that are valued by us.  If all the things we value are of this world, we will lose those as well when the son of Man comes.  If the things we value most already belong to God, we can't lose them when he returns.   We need to make sure we're living our lives in obedience to God and in communion with him so we will be ready.