–In the common greetings for holidays and other special days, we usually use the word Happy, as in Happy New Year!, Happy Valentine’s Day!, and Happy anniversary! But for Christmas, the common greeting in the United States is Merry Christmas!, NOT “Happy Christmas!”
Happy Xmas (War Is Over)?
http://en.wikipedia.org/wiki/Happy_Xmas_(War_Is_Over)
–Be Careful! We do NOT say “A happy New Year!” as a greeting on its own. We do, however, use “a” before happy New Year in a few longer expressions, such as “Have a happy New Year!” or “Merry Christmas and a happy New Year!” So be careful when writing your New Year’s cards this year!
http://www.berlitz.co.jp/wm/05/1206.htm
http://www.kunishiro.sakura.ne.jp/column/20/c25.shtml
大昔、M先生から、Aが入っていないので、間違えと言われたことを思い出しました。
and happy birthday!