In verse 7, apocalypsis is translated "coming" in the King James and "revelation" in the New King James. Paul clearly refers to the return or the second coming of Jesus Christ; he uses the word in relation to Christ appearing visibly at a specific time: His day.
This "day" of course does not refer to a specific day of the week, but rather to the period in which the misjudgment of man ends and the righteous judgment of God begins. Mankind, under the influence of Satan, has been trying in vain to rule himself for 6,000 years, or six "days," using the principle in II Peter 3:8 of one day equaling one thousand years. The seventh "day" is when God intervenes and establishes His government, so that mankind can finally understand how to live. That day begins with the visible appearance of Jesus Christ, coming in the clouds in all of His glory (Matthew 24:30; Mark 13:26).
II Thessalonians 1:7-10 speaks of that same day, or that same time:
. . . and to give you who are troubled rest with us when the Lord Jesus is revealed [apocalypsis] from heaven with His mighty angels, in flaming fire taking vengeance on those who do not know God, and on those who do not obey the gospel of our Lord Jesus Christ. These shall be punished with everlasting destruction from the presence of the Lord and from the glory of His power, when He comes, in that Day, to be glorified in His saints and to be admired among all those who believe, because our testimony among you was believed.
Here again, apocalypsis refers to the person of Jesus Christ, and it plainly describes His visible revelation—His unveiling—when He returns from heaven with His angels to take vengeance on those who do not know God and disobey the gospel. When He is revealed in that day, not only will He appear in glory, but He will "be glorified in His saints." At that time, His saints, people He has separated to Himself, will be resurrected and exchange their earthly glory for heavenly glory (cf. I Corinthians 15:40-49).