Albert Barnes Commentary Matthew 24:39

"and they knew not until the flood came, and took them all away; so shall be the coming of the Son of man." — Matthew 24:39 (ASV)

And knew not. That is, they did not know the exact time, until it came upon them. So, he says, it shall be when the Son of man shall come. They will not know the precise time until he comes, and then they will be found engaged in the ordinary business of life, unconcerned.