It's easy to know that the Rapture will happen before the Great Tribulation. We know from Daniel that the Great Tribulation will start as soon as the world ruler signs a 7 year peace treaty with Israel.

(Daniel 9:27 NIV) He will confirm a covenant with many for one 'seven.' In the middle of the 'seven' he will put an end to sacrifice and offering. And on a wing of the temple he will set up an abomination that causes desolation, until the end that is decreed is poured out on him."

Therefore, we will know the exact date of Jesus' return based on that treaty.

Yet Jesus said this,

(Matthew 24:38-44 NIV) For in the days before the flood, people were eating and drinking, marrying and giving in marriage, up to the day Noah entered the ark; {39} and they knew nothing about what would happen until the flood came and took them all away. That is how it will be at the coming of the Son of Man. {40} Two men will be in the field; one will be taken and the other left. {41} Two women will be grinding with a hand mill; one will be taken and the other left. {42} "Therefore keep watch, because you do not know on what day your Lord will come. {43} But understand this: If the owner of the house had known at what time of night the thief was coming, he would have kept watch and would not have let his house be broken into. {44} So you also must be ready, because the Son of Man will come at an hour when you do not expect him.

Jesus can't come back with no one seeing him and also come back on the clouds of heaven when everyone sees him unless he comes back twice.

That's why we need to be ready spiritually for his return because it could be at any time. We won't have a 2,520 day warning.