The pre-festal iambic Katavasiai of Sunday before Christmas, Χριστός εν πόλει..." is dated as a poetic text from the 10th -11th century. However, they were set to music by Petros Bereketis at the beginning of the 18th century. They were not recorded at the Heirmologion of Chourmouzios and thus remained on the margin of the liturgical act. In recent years prof. Grigorios Stathis and Lycourgos Angelopoulos recorded and presented these Katavasiai in digital recording and in concerts, making them known to the public. The code Gritsani 31 contains, in the last pages between the Kalophonic Heirmoi, the aforementioned Katavasiai. In new notation method they were interpreted by Grigorios Protopsaltis. The purpose of this paper is to study the interpretation of Grigorios by contrasting the two musical compositions in order to understand the palaeographical approach and interpretation of Grigorios Protopsaltis on the Sunday before Christmas Katavasiai.