If you're hesitating because, like me, you're a chicken who's terrified of rejection, then wait until January 1. But acknowledge that you're doing it out of fear, nothing rational.
Otherwise, if the query and the manuscript are ready, and the agency is open to queries, and you're not a chicken looking for excuses to delay, then just send it and forget about it. No, they probably won't read it on Christmas or Christmas Eve or even the week between Christmas and New Year's, but it'll be higher in their queue than if you wait until January to send it.
ETA: JAR was posting at the same time as I was. Didn't see his response until mine went up.