Most believe the prayer dates back to the 3rd or 4th centuries, but there is also a tradition that Nicodemus
prayed these words as he took Jesus' body down from the cross with Joseph of Arimathea. It was also used regularly at the Council of Chalcedon in 451.
While the Trisagion is central to the liturgies of Orthodox Christianity, it plays an important role in the West as well. In addition to use during Advent or Lent, these words are prayed on Good Friday, both during the Reproaches and between stops of the Stations of the Cross. The Trisagion is also incorporated into the anthem In the Midst of Life, said on Holy Saturday.Sanctus Deus
In the Eucharistic liturgy, we also hear echoes of this "thrice holy" in the Sanctus (Holy, Holy, Holy). This refrain is deeply embedded in Scripture (Isaiah 6:3, Revelation 4:8).
All of which is simply background to suggest that you use the Trisagion in your Advent devotions this year. Consider memorizing it and using these words as a mantra throughout the season. There's no right or wrong way to pray the Trisagion, but it does help to first commit it to memory (it's short!). I find it helpful to say the words silently or aloud at various points in the day, as a reminder of God's presence.Ἅγιος ὁ Θεός
It's also helpful to set aside a few moments each day to reflect more deeply upon the words. Speak each phrase slowly and then sit with it awhile. What images come to mind for you? What feelings does it evoke? You may even want to jot a few down.
The hope is that this simple but profound devotion will draw you ever nearer to the heart of the Holy One. Which, in the end, is what this season of expectation is all about.