Given your history w/ testing like this, I'm curious if you see value in asking 'why', and whether or not there are useful ways to get an answer. It seems like in addition to satisfying my curiosity about human psychology, it would be helpful in informing next-tests to make the entire process more efficient... but I don't see much talk about the 'why' when it comes to these tests.
I'm curious to know how much "Merry Christmas" improves results vs. a baseline compared to how much "Happy Holidays" may hurt results, as has been suggested by others. The linked article only compares "Merry Christmas" vs. "Happy Holidays", which makes it hard to know if "Merry Christmas" is really helping, or if "Happy Holidays" is only hurting.