Hello kind anon, thank you so much for the question ! ❤️

Tenma doesn’t have particular preference for holidays ( he would hate obon tho, the days for celebrating the dead coming back among the living… he doesn’t believe in that, the whole tradition make him feel particularly miserable… )

But for Atom, definitely Christmas ! ✨ In Japan, the celebration are kinda more couple oriented than family oriented, you eat fried chicken and a Christmas cake and spend a nice evening with your loved one, Atom would love to spent time like that with Tenma ❤️ and there’s also all the additional symbolism around Christmas, Christmas Eve is a holy night, it’s the time of miracles and be kind to each other… those notions would please Atom. He would also love to buy a Christmas tree and decorate it with his dad ❤️ I think Tenma would feel neutral about this holiday but he loves spending time with Atom and watching him getting so excited about celebrating 🥹

Now that I think more about it, Tenma would maybe prefer new year holiday, in Japan you spend it with your family and it can be very chill, just staying at home, eating some new year food and play games 💕 Atom would still insist they go to the nearby shrine for the first prayer of the year, it’s a very common thing to do and you can find your new year fortune here, Tenma doesn’t like going to shrines and temples especially in new year since it’s so crowded lol but again he would indulge Atom for this day ❤️

Thank you again for the question kind anon, take care ! 💕