.....from an article on Zenit (HT to Ellyn at Oblique House):
ROME, NOV. 29, 2005 (Zenit.org).- Answered by Father Edward McNamara, professor of liturgy at the Regina Apostolorum Pontifical University.
Q: What would you consider an appropriate time during Advent to put up Christmas trees, ornaments, lights and other decorations in churches and Christian homes? -- B.O., Lewistown, Pennsylvania
A: This question is simple only in appearance because customs surrounding the celebration of Christmas vary widely among different cultures.
From a strictly liturgical standpoint the preparations for receiving the Christ Child intensify from Dec. 17 onward and this is probably a good time to set up the parish crib, except for the image of the child, which is often added just before Midnight Mass in more or less solemn fashion.
Other parishes prefer to set up the crib on Christmas Eve. There are no official rites regarding this widespread custom.
In those places that use the Advent wreath, it is placed on the first Sunday of Advent. The Book of Blessings issued by the U.S. bishops' conference contains a simple rite for blessing the Advent wreath which may profitably be used.
Dec. 17 or the nearest Sunday might also be a good date to set up Christmas trees and other decorations in Christian homes, but it really depends on local custom and tradition. It is unnecessary, however, to fall under the spell of commercial enterprises which tend to anticipate the Christmas season, sometimes even before Advent begins. (I added the emphasis!)
Because some Christmas decorations have often lost their original religious meaning, churches should be rather circumspect about employing them and should do so with great discretion. If used at all, these decorations are best set up on Christmas Eve so as to respect the integrity of the Advent season.
Christmas trees are preferably located outside the sanctuary and church proper, and are best left in vestibules or church grounds. This has been the practice in St. Peter's Square from the time of Pope John Paul II.
As far as possible, decorations should be religiously themed, leaving plastic reindeer, sugar canes and Santa Clauses in the local shopping mall or at least within the confines of the parish hall for children's events.
Within the church proper, apart from the crib, Christmas may be evoked by using, for example, traditional poinsettias, holly and other traditional elements according to the culture.
As I mentioned, different cultures celebrate Christmas in various ways.
In some countries, such as Venezuela, many people live the novena before Christmas by attending a special "Cockcrow" Mass celebrated at 5 a.m. each day.
In Mexico, during this same period, family and neighbors often take turns in hosting a "posada," a procession in which the group goes from house to house singing a traditional song in which St. Joseph and Mary request, and are refused, hospitality until finally they are festively welcomed at the last home, which has prepared snacks and traditional games for all.
This is interesting to me, because we have an annual discussion of when to put up our tree. Our priest thinks Christmas Eve is the appropriate time, but part of that is colored by the fact that he isn't a mom in a family trying to get everything ready. We tried that a couple of years, but it left far too much to do too late. And we weren't able to turn it into the little family party that it had always been before. Then we tried the Sunday before Christmas--and that was better. Please note, we leave our tree up until Epiphany--on January 6th--lights shining brightly into the night long after our whole neighborhood has hauled their trees to the curb. We do have an artificial tree, so dryness is not a problem for us.
The 17th seems perfect to me. But we'll make FrA happy by not turning on the lights until after mass on Christmas Eve.
Oh, and last year, we kept our nativity set out until Candlemas in February. I became convinced of that when I went on pilgrimage to Rome in late January, and every single church still had theirs out. And St. Peter's life-size set was still in the courtyard.