The Gloria and the Creed are reserved for our most solemn holy days, which includes the Sunday Celebration of the Eucharist and any other holy days in the Church (including Solemn Feast Days). The General Instruction of the Roman Missal (nos. 53 and 68) states that the Gloria should always be sung (or said) on Sundays (except during the seasons of Advent and Lent). The Creed should be said or sung at all Sunday celebrations and solemnities. Why is this the case? The Gloria and the Creed are not necessary for a celebration of the Mass, and indeed are ornate ways to express the faith that we believe. Therefore, we reserve these beautiful expressions for our most solemn and festive occasions.