The Thessalonians had known about the
Day of the Lord, told many times in the Old
Testament as a time of persecution.

Paul wrote "I beseech you" , he had great
concern for them. By reason of "the coming
of our Lord Jesus Christ," and by reason of
"our gathering together unto him, That ye
be not soon shaken in mind, or be troubled,
neither by spirit, nor by word, nor by letter
as from us."

Someone had either written or told them
that the day of the Lord had begun so to
expect persecution.