In the sun He has placed His tabernacle: and He Himself like a bridegroom going forth from His chamber will rejoice like a giant to run His course

Fr Hunwicke has a superb post on Advent's clues to the theological and scriptural underpinnings of ad orientem worship.

Psalm 19 ... except that in the Vulgate and the Septuagint, the 'official' Latin and Greek translations of the Book of Psalms, it's Psalm 18 (let's not go into the different enumerations of the psalms just now) ... is central to the thinking of both East and West about the Incarnation: and several other things too. If you already know about this psalm and its 'reception' in the Fathers and Hymnographers of the Eastern and Western Churches, then it's a waste of your time to read this post. But if you don't, then please consider giving some thought to it, even though what follows is a bit dense and deep at first sight. Doing so will draw you deeply into the way in which those who framed our liturgical thinking did their theology....
Instead, here is a translation of two crucial verses of this psalm, as rendered by the Latin Vulgate and the Greek Septuagint (abbreviated to LXX), the two versions by which Christians of both traditions have have always worshipped.

5.In the sun he has placed his tabernacle: and he himself like a bridegroom going forth from his chamber has rejoiced (LXX: will rejoice) like a giant to run his course.
6. From highest (LXX: furthest) heaven {is} his going forth: and his meeting is even unto its highest (LXX: furthest); neither is there one who might hide himself from his heat.

To be as brief and plain as possible (which means slicing through some complications), our Christian forebears took the bridegroom to be Christ. The bridal-chamber is the womb of the Blessed Virgin. In that Womb he united Godhead with manhood as bridegroom is united to bride, so that he is a giant with two natures in one Person. His going forth is his eternal generation, as the Divine and Only-begotten Son, from the Father. His meeting is the Son's meeting even to an equality with the Father.

Now consider the Advent Office Hymn (from Advent Sunday until December 16 at Vespers) Conditor alme siderum. We will take the clever and accurate translation of the Anglican John Mason Neale which appears as Number 1 in the English Hymnal (the book sensible people will use if they are saying their Office in English).
Thou cam'st, the bridegroom of the bride,
As drew the world to evening-tide;
Proceeding from a virgin shrine,
The spotless Victim all divine.

And the hymn Veni Redemptor gentium ( in the Liturgy of the Hours, the hymn at the Office of Readings after December 16)
Forth from his chamber goeth he,
That royal home of purity,
A giant in twofold substance one,
Rejoicing now his course to run.

The Liturgy of the hours misses out ('ad brevitatem') the next stanza, also based on our psalm, which Neale (English Hymnal 14) renders
From God the Father he proceeds,
To God the Father back he speeds;
His course he runs to death and hell,
Returning on God's throne to dwell.

The English Hymnal does not provide the hymn Fit porta Christi pervia, which the Liturgy of the Hours orders to be said at Morning Prayer on January 1. Here is a crude prose version of the second stanza designed to show its indebtedness to Psalm 19:
The Son of the highest Father has gone forth from the palace of the Virgin, bridegroom, Redeemer, Creator, the Giant of his Church.