I don't know much about Christianity. For diwali we are saying happy diwali and for new year we are saying happy new year, but for Christmas why we are saying Merry Christmas. What is the meaning of "Merry".
"showing high spirits or lightheartedness. merry suggests cheerful, joyous, uninhibited enjoyment of frolic or festivity" http://www.merriam-webster.com/dictionary/merry What is the meaning of meaing
We feel simply bored by using the same word again and again on different occasions. So it is normal to use different words conveying the same meaning.
English has lots of words with similar or overlapping meanings. Merry has a broadly similar meaning to happy but a merry person would also show outward signs of happiness and spread that happiness to other people through cheery goodwill.
Merry to me means being a little bit drunk, not too much, just enough to be in good spirits and friendly towards other people.