I just chose the latest to greet.
Merry CHRISTmas and a Happy New Year!
The Christmas season ends on the Feast of the Three Kings (first Sunday of January of the New Year or January 6, whichever comes first), so it's inappropriate to say Belated Merry Christmas on December 26 onwards.
Also, I refrain from writing Merry X-mas, otherwise the meaning of the season disappears.