That's not too substantial, fortunately. Many of my websites/niches experience huge growth over the holidays as younger audiences have all day to socialize in my communities without school. Some of my websites have stalled growth and even reduced activity over the break.
But you'll want to find out if something mechanical changed to cause this rather than the natural fluctuations of visitor activity.
One of the main reasons for traffic to change on a whim is when your Google position for prominent keywords change. Even moving from page 1 to page 2 for one good keyword can take a toll on your traffic.
I recommend you start using a search engine position tracker for your keywords. It'll help you realize gains in any SEO-related activities you engage in, and while it won't retroactively show your past rankings, it can help give insight to future traffic.
Link Assistant's Rank Tracker is the best premium software for this, but cheaper web-based products with free trials include SEScout and SEOmoz's Rank Tracker.