Firstly, bear in mind that over the last 20 years the "social contract" between employer and employee has been dramatically modified, particularly with respect to the "at will" clauses that employers use in defining their(and your) obligations. Basically that means that 2 week notice is indeed ONLY a courtesy, because employers have themselves disclaimed the notion that they owe you anything.
As a pragmatic matter though, you might want to extend them the courtesy, if it doesn't create any difficulties for you. One thing to consider: if you don't work any days past the Christmas holiday, you likely won't get paid for the holiday. Check your employer's holiday/time off policy to see if they have anything written about this. It's not unlikely that they will receive your notice, and then say, "ok, very good, thank you...let's make December 23 your last day.
Also, do remember that they MAY BE (depending on which state) required by law to pay your accrued vacation time. They may just cut a check for the accrued time, OR they might require you to take the time in lieu of some of your notice.
The best answer probably is to go to your manager, and say "I'm giving my notice. Two weeks brings us to the end of Dec. What makes the most sense for you, to wrap up on 31 Dec, or do you need a few days past that?
Another consideration: Many HR departments like to cut you off before you work any days into the new month...because often their obligation to pay health insurance and other insurance benefits accrues on the first of the month, regardless of you only working one or two days into the month.