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The December 2011 arrivial stats are available here

http://www.bts.gov/programs/airline_.../table_03.html

you can change the year and month in the url for other periods

Keep on scrolling down to find Newark, they are listed last.

December is a tricky month at Newark, you can have many good days, and one snow or ice storm can come through and impact just a few days of travel, but a huge amount of flights. I would not use the number to say the entire month is like that, and if Newark is your destination, being late really won't impact you that much. If you are connecting to someplace else that's another story.

Airplanes are running pretty much full year round, so it won't be that much busier on December 14th than other times of the year. You will probably have more leisure travelers, and they will have more stuff with them because it's cold in Newark in the winter, and that may slow down the lines, but it won't be like there will be five times as many people.

I would get to the airport earlier, I would plan for delays, though you may not get them. Remember it can be cold, so plan on warm clothing if that's your destination. New York is a very busy place over the holidays, so the lines at customs and immigration and all that may be a bit longer and slower.

Much of it depends totally on the weather, and nobody can predict that now, they can't even get the next day right.