Yes, there are certainly similarities between the two great penitential seasons of the year: the emphasis is on the need for contrtition, and there really should be no weddings scheduled. And, depending on your tradition, some other things are off limits too. Like certain foods. An Advent fast is urged just as a Lenten fast is urged. There are venerable traditions which would agree, and a particular Advent fast is a long-standing custom in some places.
It seems to me, though, that fasting ought at least to be enjoined a bit more seriously during Lent than during Advent; the former is, after all, is even called Die Fasten in German.
The seasons have significant differences, too, which, I would submit, can give us some direction as to how they are to be handled respectively. Lent is a season which tends to grow darker as it moves on, until the darkest time, the Triduum Sacram, is reached. Then, suddenly, at the Great Vigil, Easter arrives and all is instantly brightened. Advent, by contrast, tends to grow lighter from week to week, as is most prominently seen in the lighting of successive Advent candles each week. Advent is a season of penitential hope and expectation, while Lent is a season of penitential sorrow as the Day of Christ's passion approaches.
Inasmuch as Advent has this 'brightening' character to it, it seems prudent to me to allow that certain customs need not be quite as stringently applied as in the case of Lent. OK, so for example, neither season ought to celebrate a wedding, but I for one don't mind if people want to have a Christmas party during Advent. Our choir has one, for that matter.
If it makes you liturgically uncomfortable to sing Christmas carols at parties or in the marketplace before Christmas, I recommend remembering that this isn't church. The Advent hymns can and should still be the standard fare for Holy Mass and any other worship settings; but I'm fine with carolling outside of Mass. After all, it's the only time of year you can actually hear some great hymns about Jesus in the marketplace at all. They want to pipe in Hark, the Herald for shoppers during December? Hey, fine with me! I'll take it any time. That goes for creche scenes and holiday lights, too. I love 'em all! Holiday shopping season have too much tinsel, you say? Nay, I say, bring it on. Don't want to see too much of the Babe in swaddling clothes? Well, I do, and that image is as meaningful to me as the crucifix. Both are saying, Pure Gift from Heaven. Now what can be wrong with that message?
So, at our church, we, um, gradually decorate during December. This is a bit parochial, perhaps (well, come to think of it, maybe not), but we put up a little more each week. The tree goes up some time in the middle of the month. We hold off on the gazillion poinsettias until Christmas Eve, and also the white paraments, and, yes, the Chrsitmas carols. Nevertheless prior to Christmas Eve there's a hint of a difference in liturgical appearance from week to week, consistent with the lighting of another candle each week.
So just try to think of those Christmas parties as times to rehearse the carols meant for Christmas itself. OK? How's that? We're just practicing, see? It's all good!