The prayer known as the "Phos Hilarion" (gladdening light) dates back to the 3rd or 4th Century CE.  It was a hymn meant to be sung at the setting of the Sun, as the lamps of the evening were lit.  In a world without the bright electric lights we enjoy today darkness seemed to almost overwhelm human existence.  Yet, in that moment where the darkness of night might lead to despair this prayer was sung as a way of saying, "The light of our Triune God shines even in the darkness."
It's Triune nature is evident in the words, equally praying the Father, Son, and Spirit.  The verb "hymn" means "praise."  This use goes back to the book of Psalms where song marked "hymns" were songs of praise for the glory of the Lord.  In a sense, a "hymnal" is a "book of praise."
For the remainder of Advent, say this prayer with with your family in the evening, and allow the Light of God to brighten the darkness.  There is an excellent contemporary version that you can download through Amazon, a more traditional version is also available.