Christmas is a national holiday in Korea and the many Korean Christians do celebrate the day. In addition, the holiday season comes to stores across the nation, just as in the West, though shopping for gifts surely does not reach the frenzied level you'll find in places like the US.
Having said that, Koreans don’t generally hold Christmas parties.
Instead, Koreans are much more focused on the new year, and every self-respecting Korean attends several end-of-year parties (called 송년회 or 연말 모임) each year. These get-togethers are generally held on a personal basis and for business throughout the month of December (not much at all happens on December 31, though). Because the end-of-year gatherings are frequently scheduled months in advance, it can be hard to arrange evening meetings with several people at once during this time because of prior commitments, so try to plan as far ahead as possible if wanting to meet Koreans on business in December.
I'll also point out that because of all the partying (and other factors, such as not knowing what sudden administrative changes the new year will bring), very little business gets done in Korean companies in December, and no important outward-focused decisions at all get made during this navel-gazing time.
Koreans don’t commonly wish each other “Merry Christmas” (though this greeting is becoming more popular, and it is usually expressed in English). Instead, they focus on the new year… Both the Western new year on January 1, and the lunar new year (aka Chinese New Year) in late January or early February. Thus, new year’s greetings are often given twice!
You can't go wrong wishing Koreans a “Happy New Year” and there's really only one way to say it in Korean: “Sae hae bok mani baduseyo” (새해 복 많이 받으세요)
However, in writing, "Happy New Year" can be expanded out in many ways. For ideas, check out these collections of holiday greetings in Korean.