I won't comment about China or Japan but yes, Christmas is celebrated in full force in South Korea. Till the 70s, S.Korea was largely atheist with second majority of Buddists (although officially, S.K is Buddhist).
However, the influx of Evangelics in S.K brought about by the US cultural dominance has literally changed the entire landscape of South Korea. When I say literally, then it is quite the case. Anyone who had landed in Seoul, Busan or other Korean cities will note that almost every roof of a house has a cross which denotes a Christian following. When I first visited Seoul in 1985, its television and radio broadcasting was totally in control of the US forces. English language television news (if I remember it was KRBC) had a US uniformed officer reading world news. Access to or around US bases to South Koreans per se was only by virtue of South Korean women married to American husbands. If you happened to be a Korean male, you didn't get any entry.
A lot has changed since the 80s especially. South Korea has gone more vibrant and more independent of US military control over its day to day affairs (which is also a result of the crashing down of the Iron Curtain cold war).
Christianity is on the spread in S.Korea and in the near future, one may expect at least 70% of the population converting to the word of the Lord.