As the actual time of the rapture was not explicitly mentioned in the Bible, I would really like to believe that it is pre-tribulation, but to be honest, I am more inclined to believe that the rapture will be during (mid) the great tribulation and before the bowl judgments...the actual rapture ("Reaping the Earth's Harvest") was indeed mentioned in Revelation14
14 Then I looked, and behold, a white cloud, and on the cloud sat One like the Son of Man, having on His head a golden crown, and in His hand a sharp sickle. 15 And another angel came out of the temple, crying with a loud voice to Him who sat on the cloud, “Thrust in Your sickle and reap, for the time has come for You to reap, for the harvest of the earth is ripe.” 16 So He who sat on the cloud thrust in His sickle on the earth, and the earth was reaped.
Thus the great tribulation was shortened for the elect, Matthew 24:22
And unless those days were shortened, no flesh would be saved; but for the elect’s sake those days will be shortened.
And then this will be followed by "Reaping the Grapes of Wrath" which is the pouring of the 7 bowl judgments
17 Then another angel came out of the temple which is in heaven, he also having a sharp sickle.
18 And another angel came out from the altar, who had power over fire, and he cried with a loud cry to him who had the sharp sickle, saying, “Thrust in your sharp sickle and gather the clusters of the vine of the earth, for her grapes are fully ripe.” 19 So the angel thrust his sickle into the earth and gathered the vine of the earth, and threw it into the great winepress of the wrath of God. 20 And the winepress was trampled outside the city, and blood came out of the winepress, up to the horses’ bridles, for one thousand six hundred furlongs.