By times and seasons then, before mentioned, he meant the time: of the Lord's coming, or he applies what he spoke in general to this particular, which he here calls the day of the Lord. And though they knew not the particular time, yet they did know this, it would be sudden and unexpected, coming as a thief in the night, Revelation 16:15: the comparison is to be restrained only to the suddenness of it; for his coming will be welcome, and so not as a thief, to all that believe. And it is called the day of the Lord here and elsewhere, 1 Corinthians 3:13 Philippians 1:6,10, and that day, 2 Timothy 1:18, not to be taken for a natural day, but a certain period of time. Any eminent manifestation of God, either in works of mercy or judgment, is called his day in Scripture, Isaiah 2:12 Jeremiah 46:10. And so because Christ will be more eminently manifested now than ever before, therefore his coming is called his day; and that it would be sudden they did not only know, but know perfectly, or accurately; Ephesians 5:15, circumspectly: there could be only conjectures about the particular time: the influence hereof was powerful upon their hearts, and so they may be said to know it perfectly. In religion, knowledge is not perfect which is not operative.
Related Commentaries of 1 Thessalonians 5:2
1 Thessalonians 5:2
2 For yourselves know perfectly that the day of the Lord so cometh as a thief in the night.