The reason for using this phrase is the abundance of the season spanning late November to mid-January in different holidays of different religions (such as Christmas for Christians, Chanukkah for Jews, etc.). Since you cannot always know what faith the recipients of your greetings are, "рПЪДТБЧМСА У РТБЪДОЙЛБНЙ!" would be the safest thing to say or write, without being specific as to which holidays you are talking about.