Humans developed holy days and rituals around the observable events and changes in their environment—solstice and equinox, the passing of comets, the changing of seasons, the migration of animals. When we go to space, we’ll be leaving behind us everything that made for the underpinnings of our social ritual. Will those things mean more or less when we take to the stars?
In our Pandora Project stories, we’ve suggested that they may mean more, that we may structure our new life around the relics of the old one. We’ll build arcologies with seasons, organize the rhythms of our years according to the old celebrations. Long after anyone remembers what it was like to have Yuletide in the snow or Ostara in the spring, we will be imitating and perpetuating those familiar—and familial—events.
What does Christmas look like in space? Do we light up buoys outside the spaceport with seasonal lights? We hope so, lights are one of our favourite parts of the winter holiday. We imagine that, in our Pandora Project future, the customs will be upheld just as they are now.
For Saturnalia, narrow corridors of a cramped mining vessel are strung with bright lights in garish colours, the old traditional candle flame shape, and fake greenery garlands have been draped around the round portals that lead from one part of the ship to another. Plastic mistletoe hangs at the entrance to the cockpit and the captain, feeling the spirit of the season, insists on kissing everyone under it.
A mall on Luna opens up a nivitorium, a domed wonderland where ultrasonic crystal generators create snowstorms that fall every hour on the hour. Real evergreens, imported from Earth, are draped with lights and hung with glittering glass balls. Children are bundled in rented snowsuits and sent out to play, dragging sleds up sculpted hills the way their own parents did, and the grandparents watch from an observation deck above. Even they don’t remember an Earth where it snowed.
The giving of gifts will always be a part of any human culture. Students at military academy, training to fly space fighters, exchange Saturnalia gifts. On the twenty-fifth of December, the upperclassmen serve the juniors and the commanding officers wait tables.
Casinos overflow with patrons who would not usually gamble during the year. On space stations and colonies, public meals are held for everyone to share. For this one day of the year, everywhere except where it would be deadly to do so, real candles are lit to mark the return of longer days that will come with a shift in the lighting programs instead of the return of the sun.
Contact – Infinite Futures 
Just a caveat on lighting up buoys in Christmas colors–starboard is green and port is red, very important navigational colors that you don’t want to confuse with standard Christmas colors.
Of course, a GM of mine thought to replace the porchlights flanking his garage door with green and red bulbs for the holidays one year and as luck would have it, they aligned perfectly with starboard-green/port-red relative to the bonus room window above, where we regularly gamed. (Kinda like a spacehip’s cockpit window). Totally accidental on his part, btw, but I thought it was delightful.
One of my neighbors has done exactly what you described with the porch lights around their garage. It’s always very fun to drive by. 🙂
As for open flames in closed atmo environments? Yikes. Depending on the oxy levels present in the atmo mix, it could get a mite dicey!
That’s why it came with the caveat “everywhere except where it would be deadly to do so.” There are colonies and space stations as well as spaceships in our Pandora Project stories, so there are a lot of options for safe candle use. 🙂