We read over and over in the New Testament the reference to the “Day”. It usually refers collectively to the day of judgment and the return of Jesus Christ. Often there is a reference to the Day of Atonement, or as the writer of the book of Acts simply states, “the Fast” Acts 27:9. “Day of Atonement” is translated from the Hebrew Yom Kippur, which means “day of covering”. This is the same term used in Exodus 25:17 for the mercy seat that covered the Ark of the Covenant which contained the written Law of God. This reminds us that our transgressions of the Law (which John tells us in 1 John 3:4 is sin) are covered by God’s mercy or atonement.
In Hebrews, there is an admonition to assemble together and “(exhort) one another...so much more as you see the Day approaching.” Hebrews 10:25 This is a reference to the “Day of the Lord”–the time of Christ’s return, and resurrection and judgment of the church. (See Isaiah 2:12) Paul told the Thessalonians to “edify one another” , “that this Day should (not) overtake you as a thief.” 1 Thessalonians 5:4,11 Jesus tells us to “take heed to yourselves, lest...that Day come on you unexpectedly...Watch therefore, and pray always that you may be counted worthy to escape...and to stand before the Son of Man.” Luke 21:34-36