To understand why the Church changes colors on the third Sunday of Advent we’ve got to understand what all these colors mean in the first place and why we use them. The Church follows a liturgical calendar that begins on the first Sunday of Advent and ends on the Solemnity of Christ the King. The liturgical year is made up of different seasons and each season has a specific color connected with it. Advent and Lent are primarily purple, a color that symbolizes repentance. The Christmas and Easter seasons use white to represent light and glory. White is also used throughout the year on feasts of Mary and other saints who were not martyred baptisms, weddings and Eucharistic celebrations. Ordinary time, the rest of the year, uses green to call to mind life, growth and hope. Red is used on specific feast days to symbolize the blood of martyrs or Christ (Good Friday) and also the fire of the Holy Spirit (Pentecost).
Rose may only be used on two Sundays in the liturgical year: Guadete Sunday, the third Sunday of Advent, and Laetare Sunday, the fourth Sunday of Lent. The names are taken from the first few words of the introductory psalms for those liturgies. These Sundays are the midpoints of Advent and Lent, two seasons focused on repentance, fasting and preparation. The Church has designated these two Sundays to call us to remember that even in the midst of fasting and penance we are to be filled with joy. We are called to take a respite from our solemn journey and call to mind the joy of our salvation.