“But of the times and the seasons, brethren, ye have no need that I write unto you. For yourselves know perfectly that the day of the Lord so cometh as a thief in the night.” 1 Thessalonians 5:1-2 (KJV)
Paul is writing the Church about something now that they had already learned; “…You have no need that I write unto you” concerning this matter.
The Lord’s day is the first day of the week [Sunday]. The “Day of the Lord” is a time of judgment coming on the earth unlike anything that has ever happened, neither will anything like ever occur again.
“…Comes as a thief in the night” is coming when it is least expected, and it is something no Christian should be alarmed about, neither afraid. It could just be that the rapture [catching away] of the Church is what sets off the “Day of the Lord”, and then “The time of Jacob’s troubles” will begin (Jeremiah 30:7).
If you have never asked for the love, and grace of God from Calvary’s cross through the death, burial and resurrection of Jesus Christ to save you that horrible event is in your future. Repent of your sins, turn to Jesus Christ and be saved.