There were times when the persecutions
grew strong. The people were worried and
wrote to Paul asking "Is this the Day of the
Lord?"

The Day of the Lord is a well-known set of
circumstances given in the New Testament.
Paul wrote back saying, "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