This can only happen when December 17 falls on Gaudete Sunday. (And how much rose did you see in church?) It's as if the change in color is also highlighting another aspect of the liturgy which will be celebrated later. It seems to say, "Your rejoicing is about to become complete."
All this week, the last "full" week of Advent this year, the Liturgy of the Hours will include some of the most beloved chanted prayer the Church can offer.
From the website of the United States Conference of Catholic Bishops:
The Roman Church has been singing the "O" Antiphons since at least the eighth century. They are the antiphons that accompany the Magnificat canticle of Evening Prayer from December 17-23. They are a magnificent theology that uses ancient biblical imagery drawn from the messianic hopes of the Old Testament to proclaim the coming Christ as the fulfillment not only of Old Testament hopes, but present ones as well. Their repeated use of the imperative "Come!" embodies the longing of all for the Divine Messiah.As has been this 'blog's tradition, for the next seven days I offer my own very humble reflections upon these liturgical passages. The format has not changed much over the past few years; it has, in a way, become an on-line booklet. It is my hope that my meager musings will give your soul cause to reflect ever more deeply upon the great mystery to be celebrated. May they continue to be a means to finishing your Advent worthily.
In two hours, they begin.