The English word advent originated from the Latin word adventus, meaning arrival, or coming. The already deeply-divided Christian-professing world is further split by focusing on one or the other of the two advents of the Christ, His birth or His return, one as matter of Bible History, the other as a matter of Prophecy i.e. some look to the past, some look to the future. For some of those who focus on His birth, "Advent" has become an annual tradition which is generally observed for the period during the four Sundays before Christmas - even though the date of Christ's birth is not recorded anywhere in The Bible, and even though many theologians and historians recognize the fact that Jesus Christ was almost certainly not born on December 25, or anytime at all in the winter (one of the most well-known "Christmas" verses of the Bible, "there were shepherds out in the field, keeping watch over their flock by night" (Luke 2:8 RSV) ironically proves that Christ was not born in the winter - shepherds in Israel then, and now, do not leave their flocks out in December, but rather are all brought in well before the cold winter rains, and sometimes snow, begin), but rather He was born in early autumn - thereby making December the time of His conception, not His birth.
The First And Second Coming Of The Messiah
The Son of God came the first time to be sacrificed as "the lamb of God." The Son of God is coming the second time to rule the world with absolute power and authority.
The first coming:
"In those days a decree went out from Caesar Augustus that all the world should be enrolled. This was the first enrollment, when Quirinius was governor of Syria. And all went to be enrolled, each to his own city."
"And Joseph also went up from Galilee, from the city of Nazareth, to Judea, to the city of David, which is called Bethlehem, because he was of the house and lineage of David, to be enrolled with Mary, his betrothed, who was with child. And while they were there, the time came for her to be delivered. And she gave birth to her first-born son and wrapped Him in swaddling cloths, and laid Him in a manger, because there was no place for them in the inn."
"And in that region there were shepherds out in the field, keeping watch over their flock by night. And an angel of the Lord appeared to them, and the glory of the Lord shone around them, and they were filled with fear. And the angel said to them, "Be not afraid; for behold, I bring you good news of a great joy which will come to all the people; for to you is born this day in the city of David a Savior, who is Christ the Lord. And this will be a sign for you: you will find a babe wrapped in swaddling cloths and lying in a manger."
"And suddenly there was with the angel a multitude of the heavenly host praising God and saying, "Glory to God in the highest, and on earth peace among men with whom He is pleased!" (Luke 2:1-14 RSV)
The second coming:
"Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light, and the stars will fall from heaven, and the powers of the heavens will be shaken [see Signs In The Heavens]; then will appear the sign of the Son of man in heaven, and then all the tribes of the earth will mourn, and they will see the Son of man coming on the clouds of heaven with power and great glory; and He will send out His angels with a loud trumpet call, and they will gather His elect from the four winds, from one end of heaven to the other" (Matthew 24:29-31 RSV)
"Then The Lord will go forth and fight against those nations as when He fights on a day of battle. On that day His feet shall stand on The Mount Of Olives which lies before Jerusalem on the east; and the Mount of Olives shall be split in two from east to west by a very wide valley; so that one half of the Mount shall withdraw northward, and the other half southward [see Earthquake!]. And the valley of my mountains shall be stopped up, for the valley of the mountains shall touch the side of it; and you shall flee as you fled from the earthquake in the days of Uzziah king of Judah. Then The Lord your God will come, and all the holy ones with Him." (Zechariah 14:3-5)
"For The Lord Himself will descend from heaven with a cry of command, with the archangel's call, and with the sound of the trumpet of God. And the dead in Christ will rise first; then we who are alive, who are left, shall be caught up together with them in the clouds to meet The Lord in the air." (1 Thessalonians 4:16-17 RSV) [see Resurrections]
"And The Lord will become king over all the earth; on that day The Lord will be one and His Name one." (Zechariah 14:9 RSV) [see King Of The Mountain]
Fact Finder: Were the first and second comings of the Messiah prophesied as far back as the Old Testament?
See First And Second Comings In Isaiah