Along with Psalm 136 (LXX numbers as 134 & 135 respectively) is called the Polyeleos or translated to "Many Mercies", named such after the refrain used "for His mercy endures forever". The Polyeleos is sung at Orthros (Matins) of a Feast Day and at Vigils. In some Slavic traditions and on Mt. Athos it is read every Sunday at Orthros.
On Mt. Athos it is considered one of the most joyful periods of Matins-Liturgy, and the highest point of Matins. In Athonite practice, all the candles are lit, and the chandeliers are made to swing as the Psalms are sung, it is also accompanied by a joyful peal of the bells and censing of the church, sometimes with a hand censer which has many bells on it.
At vigils, it accompanies the opening of the Royal Doors and a great censing of the nave by the Priest(s) or Deacon(s).