The Tribulation comprises the hardships Christ’s disciples will endure until the Rapture.
“For those times will have tribulation such as has not been since the beginning of God’s creation until now, nor ever will be. If the Lord had not shortened those days, no one would be saved; but for the sake of the elect whom he chose, he did shorten the days.” (Mark 13:19-20)
“Prepare for Tribulation,” physically and spiritually. The Warning and the Illumination of Conscience ― an outpouring of God’s infinite Mercy and Grace for humanity ― are central to the spiritual preparation.
The Rapture will occur at Christ Second Coming when His angels will gather His elect to meet Him in the clouds.
“But immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light, and the stars will fall from the sky, and the powers of the heavens will be shaken. And then the sign of the Son of Man will appear in the sky, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of the sky with power and great glory. And He will send forth His angels with a great trumpet blast, and they will gather together His elect from the four winds, from one end of the sky to the other. (Matthew 24:29-31)