Occasionally, I have come across the assertion that Advent is not penitential.
I had always taken it as self-evident that Advent is penitential, and seem to recall that I was taught that from my earliest years.
The liturgy seems to suggest as much, not least because of the parallels with Lent (Purple vestments, one Rose Sunday by way of easing the discipline), no Gloria, and so on.
However, in order to answer this question more intelligently I got out my (old, EF) missal (my new OF one has nothing to say on the subject).
Here is what my old missal says:
The Ecclesiastical year is reckoned to begin with the First Sunday of Advent, the Sunday namely which is nearest (either before or after) to the Feast of St Andrew (30 Nov.)
By Advent (Coming) is meant in the Liturgy of the Catholic Church the season of preparation for the Festival of Christmas. The services of the Church during Advent (except on Saints' days) are of a penitential character. The Hymn Gloria in excelsis is not said at Mass; the colour of the sacred vestments is purple or violet; the altar is left unadorned; and the playing of the organ is restricted.
So it seems that traditionally, Advent was seen as a time of penance: and indeed that makes sense. Just as our celebration of the Holy Eucharist begins with a penitential rite (Confiteor, Kyrie) to help us to make ready for Our Lord's coming, so the Church's year starts with a penitential period, to help us make ready for His birth.
I also note that in our diocese, just as in Lent, our good and holy bishop has arranged for Confession to be made available on every Wednesday evening in every Church and Chapel. So history, logic, and our local bishop's latest initiative seem to me to support the proposition that Advent is indeed penitential.
@Part Time Pilgrim thought I had only proved that Advent *used to* be penitential in character. I am not sure I agree.
However, I did a little further research and find in the current General Instruction of the Roman Missal
Offices and Masses for the Dead.
§ 347. Ritual Masses are celebrated in their proper colour, in white, or in a festive colour; Masses for Various Needs, on the other hand, are celebrated in the colour proper to the day or the time of year or in violet if they have a penitential character, for example, nos. 31, 33 or 38; Votive Masses are celebrated in the colour suited to the Mass itself or even in the colour proper to the day or the time of the year.
That seems to me to imply that Advent is still regarded as a penitential season.