Luke 21: 7"Teacher," they asked, "when will these things happen? And what
will be the sign that they are about to take place?"

Luke 21 is somewhat different than Matthew 24 and Mark 13 as the parable
is for the Fig Tree (Israel) and all the trees (nations or gentiles). The
following is described as birthpangs or the beginning of the sufferings in
Matthew.

 Luke 21:10Then he said to them: "Nation will rise against nation, and
kingdom against kingdom. 11There will be great earthquakes, famines and
pestilences in various places, and fearful events and great signs from
heaven.

While Matthew 24 and Mark 13 deal with his return at end of the
Tribulation, Luke 21 says when these things "begin" to take place your
"redemption" is drawing near.

  25"There will be signs in the sun, moon and stars. On the earth, nations
will be in anguish and perplexity at the roaring and tossing of the sea.
26Men will faint from terror, apprehensive of what is coming on the
world, for the heavenly bodies will be shaken. 27At that time they will
see the Son of Man coming in a cloud with power and great glory. 28When
these things begin to take place, stand up and lift up your heads,
because your redemption is drawing near."

And what is the Redemption??

1 Cor.1:30 30It is because of him that you are in Christ Jesus, who has
become for us wisdom from God-that is, our righteousness, holiness and
redemption.
Eph 1:7 7In him we have redemption through his blood, the forgiveness of
sins, in accordance with the riches of God's grace
Eph 4:30 30And do not grieve the Holy Spirit of God, with whom you were
sealed for the day of redemption.
Col 1:14 13For he has rescued us from the dominion of darkness and brought
us into the kingdom of the Son he loves, 14in whom we have redemption,[a]
the forgiveness of sins.

Luke 21:35For it will come upon all those who live on the face of the
whole earth. 36Be always on the watch, and pray that you may be able to
escape all that is about to happen, and that you may be able to stand
before the Son of Man."