It’s about the destruction of the Temple in 70AD.
Jesus’ words about no one knowing the day or the hour was in the Olivet Discourse (Matthew 24, Mark 13 and Luke 21), where he prophecied the destruction of Jerusalem, the destruction of the Temple, the end of Jewish Temple worship and the death of hundreds of thousands of Jews.
He tells the disciples to be sober and alert, so that when the day comes they will not be caught off guard. If God had revealed the exact day and hour when it was to take place, the disciples would have gotten lazy, thinking that they had enough time to escape. They would have gone about their day to day business until the last minute. By then Jerusalem would have already been under siege by the Romans and it would have been too late to escape. Jesus does not want them to wait until it is too late to escape.
In (those) days before the flood, they were eating and drinking, marrying and giving in marriage, up to the day that Noah entered the ark. They did not know until the flood came and carried them all away. So will it be (also) at the coming of the Son of Man. (Matthew 24:38-39)
As it was, all the Christians escaped from Jerusalem prior to it’s destruction, because they stayed sober and awake, looking for the clear signs given to them by Christ in Matthew 24, Mark 13 and Luke 21. They were looking for the signs, and recognized them when they began to appear, and escaped before the siege of Jerusalem started - before it was too late.