Luke 17:24
For as the lightning, that lighteneth out of the one part under heaven, shineth unto the other part under heaven; so shall also the Son of man be in his day. For as the lightning, that lighteneth out of the one part under heaven, shineth unto the other part under heaven; so shall also the Son of man be in his day.
For as lightning lightens out of one part under heaven and shines to the other part under heaven, so the day and coming of the Son of man will be unmistakable and visible across the whole world. His return will not be a hidden event announced only in a particular place. Readiness for it is required.
The verse belongs to Jesus' teaching about the day of the Lord, the day of the Son of man. Its surrounding warning points to the judgment that came when Noah entered the ark after years of warning, and to the days of Lot, when some heeded the warning while others mocked.
That day comes like a thief in the night to all who are not watching and praying and working. The verse therefore calls people to be ready by watching, praying, and working, with those who have the light following in the footsteps of Jesus and working out their own salvation with fear and trembling.