So, let's say, "thief in the night" means, He comes suddenly and no one other than the Father know which day.
1. What difference does it make, whether one watches or does not watch? I suppose there must make a difference to watch, since Jesus emphasized to watch, right?
2. What does He mean by watching? Like keep looking at sky?
2. What difference does it make, to know, that He comes like a thief without anyone knowing?
Watching is not looking at sky. It's looking after your heart.
Philippians 1
9 And it is my prayer that your love may abound more and more, with knowledge and all discernment,
10 so that you may approve what is excellent, and so be pure and blameless for the day of Christ,
11 filled with the fruit of righteousness that comes through Jesus Christ, to the glory and praise of God.
Luke 21:34
34 "But watch yourselves lest your hearts be weighed down with dissipation and drunkenness and cares of this life, and that day come upon you suddenly like a trap.
1 Peter 5:8
8 Be sober-minded; be watchful. Your adversary the devil prowls around like a roaring lion, seeking someone to devour.
Matthew 25
31 "When the Son of Man comes in his glory, and all the angels with him, then he will sit on his glorious throne.
32 Before him will be gathered all the nations, and he will separate people one from another as a shepherd separates the sheep from the goats.
Acts 3
19 Repent therefore, and turn again, that your sins may be blotted out,
20 that times of refreshing may come from the presence of the Lord, and that he may send the Christ appointed for you, Jesus,
21 whom heaven must receive until the time for restoring all the things about which God spoke by the mouth of his holy prophets long ago.
Acts 17
30 The times of ignorance God overlooked, but now he commands all people everywhere to repent,
31 because he has fixed a day on which he will judge the world in righteousness by a man whom he has appointed; and of this he has given assurance to all by raising him from the dead.