Happy vs Merry

Today is Easter so people will be wishing each other a Happy Easter. At Christmas time people wish each other a Merry Christmas. Why is Christmas merry while all other holidays are happy? I tried to look up the answer and there are a lot of definitions for happy being an emotion and merry being a state of being. After reading a few differing opinions, I came to my own conclusion. We say it because that is how we learned to say it.