Why Merry Christmas?

I'm neither Hindu, Muslim nor Maccabee so, when my friends of those persuasions in the diaspora wish me Shubh Diwali, Eid Mubarak or Hanukkah Sameach, respectively, I don't know how to react. Many of my fellow atheists just wave back then revert to their phones as if there's something important on there.

Christians, usually of the melanin deficient variety, tread on eggshells around whether to say Merry Christmas to their melanin-enhanced colleagues without triggering an excursion to human resources. "Is it insensitive if they're not Christian?" or "It's just an harmless alternative to 'Happy holidays'," and the ubiquitous, "Why did they come to this continent if they don't like the culture?" are the typical refrains.

I omitted Christianity in my list of superstitious exclusions deliberately, though it's captured in my declaration of atheism, to highlight that nowadays, emigrants to North America, Europe and Australia, typically neither White nor Christian, don't dash about the place presumptively wishing sundry White acquaintances the blessings and salutations of their own faiths. They/we refrain because of a respect for people's individuality. Consider a turn of the tables: if you're Christian, you'd balk if a Hindu rushed up to you in October and gushed out Diwali wishes in homage to the festival of lights that celebrates Lord Ram's return to Delhi after a fourteen-year exile. In March, he would douse you in colourful powder for Holi, the very secular celebration of the triumph of good over evil, though you might wish secretly that it was accompanied by his delicious cuisine. In return, he doesn't expect to join in the cult-like birthday party of an obscure Essene deity born two millennia ago to a virgin in a Roman colony, merely because he emigrated in modern times to your Euro-centric one. Neglect that your so-called ethnic friend doesn't query the contradiction of your deity's promise to return within the lifetime of his contemporaries and trigger Armageddon, yet here you are still seeking succour at the teat of salvation in anticipation of that same Armageddon that ought to have begun already if He was to be believed the first time, and wishing everyone a Merry Christmas.

My questions continue to be: why is it necessary to celebrate Christmas, and is everyone on Holiday in December in a 365-day economy?