As the grand ol' melting pot, there are so many traditions. Here's our from the midwest US:

My family is a bit odd (or cheap depending how you look at it!), and we buy Christmas gifts all year long. Black Friday (for me) is typically the day to finalize the Christmas gift lists. (That way you go into December a little less stressed because all the gifts are bought.)

We start decorating the weekend after Thanksgiving (never sooner). We've always used fake trees as Mom and I have allergies. Although lately Dad's been buying fresh, so the tree goes up in mid-December. Everyone helps with the decorations (both inside and outside) and we've always decorated with ornaments that meant something throughout the years (this one signifies what happened in 2001, etc...)

We used to celebrate St. Nick day, and I will still sometimes (but I always seem to confuse the dates for St. Nick and Pearl Harbor day--go figure). There are always tons of stuff to do in December around the community. We like to go on more rides in the evenings and "rate" Christmas light decorations.

Companies typically have some sort of Christmas party (formal or informal).

If I have an advent calendar on hand, I'll use it; but I'm quite the forgetful person and typically open 5 windows at a time!

We go to Christmas Eve services every year. These are usually informal, candle-lite services. If we can get a group together we'll go carolling the week before Christmas as well. We do typically do our traditional Lite drive with hot chocolate and cookies driving around looking at all the cool lights.

We've always made candy and cookies together as a family and pass these out as Christmas gifts to everyone (co-workers, friends, mailman, etc.)

We don't typically open gifts on Christmas Eve--only Christmas morning. We've always been allowed to open our stockings on our own; but have to wait for the whole family to awake/arrive before we open the gifts under the tree. (My sister and I always wake up early to make monkey bread for the whole family). We'll then go around and open gifts one at a time (this can go quite long depending upon how many family members make it: one year it took us over 6 hrs!)

Then it's back to work again the 26th or 27th depending on the weekday or job.

Hope that helps!
Jo