It was around my son’s second winter on this earth, and that stressful time known as the run-up to Christmas, that I really knew. He was autistic. My boy, who loved to flip anything on wheels belly-up so he could watch them spin, his little fat hand expertly rolling them faster. My boy, who spoke in long, elaborate phrases, many of them lifted verbatim from his favourite books and TV shows. My boy, who appeared not to notice other toddlers passing in buggies, but was mesmerised by carpet fluff, sirens, the moon. My boy, who cared not one fig for Santa, but who, when we dragged the tree into the house, shedding the customary trail of needles and pine perfume, rubbed up against it like a cat, embraced it as if it were a long-lost friend who had come for tea.

Autistic children hate Christmas, or so I thought. It’s the sensory overload. The unwieldy family visits seasoned with “melancholysteria” (a festive hybrid of emotions that tastes like purple Quality Streets, hungover Victoria Wood specials and childhood) when everyone is wearing silly hats and (mostly, surely, please God) trying their best. It’s the bewildering demand to sit at the table for hours and consume 5,000 calories of enormo-bird and suspicious items collectively known as trimmings. It’s the hell on earth otherwise known as the Christmas market.

But maybe that was me, not my son.

It turns out that many things I thought about autism were twaddle. It wasn’t that my son hated Christmas: it was that he cared deeply about certain aspects of it, while others aroused zero interest or induced abject horror. Which is not far off how many of us feel about this got-to-be-most-wonderful time of the year. One Damascene moment came at the Edinburgh Christmas market, possibly as I was exchanging many pounds for a tiny tankard of mulled wine (plus extra for tankard insurance in case I nicked it; there’s the spirit of the Christmas market right there). This was during the bizarre period in the era of parenting your first baby when you don’t know what the hell you are doing, end up doing what you think you should be doing and then spend the whole time thinking: “What am I doing here?” Well, my son responded to the sights, sounds, smells, lights, queues and endless wooden-arts-and-crafts stalls as if he were my daemon. He screamed. Tears poured from his eyes like fake snow from the tinsel-wrapped dome on George Street. He smashed his head off the back of his buggy over and over again. We left immediately. We have never been back.

Children change everything, even more so when they have additional needs. So along with real life, Christmas was changed for good. And I was changed, too. The next year, I wasn’t disappointed that my son wasn’t harassing me for presents for weeks on end. I knew him, and us, better. There was less fear and more delight. I remember on the way home from nursery one December day, my son pointed at a small plastic fish in a charity-shop window. In the next window, I showed him a Christmas tree, and, in a phase when he was obsessed with defining things negatively, according to his own idiosyncratic system of what they were not, he replied: “It’s not a fish!” For at least two more years, every time we saw a Christmas tree one of us would say: “It’s not a fish!” and fall about laughing. I know, the levels of you-had-to-be-there are off the scale, but this is the way with autism. It’s hard to explain, and highly specific. The point is, even with all the challenges – and there are many – if you just open the door of your mind to autism, the world is reborn. And it’s incredible.

Some years on, with an autism diagnosis, another child and a few more Decembers under our belt, Christmas has evolved. There are no visits to meet Santa in his grotto, no dragging the kids around the shops, no big pub lunches with friends and their children. Instead, there is walking in quiet, thickly frosted nature. Low-key visits with those who love and understand us. Differently tailored but fundamentally similar experiences for our autistic son and neurotypical daughter.

On the day itself, while we eat Christmas dinner, our son will roam around, working his way through his usual fare of chicken dippers, cheese, apple slices, yoghurt and red fruit. The presents will be opened over many days in a kind of slow travel of present-giving and will be geared towards his special interests, not what we think he may or, worse, should like. One year, he was obsessed with traffic lights and asked for “the green man”, which took some work. This year, latex gloves are a big thing and his face when opening a bumper box of them will be priceless. Every year, without fail, he asks for a Yoyo, one of those 60p rolls crafted out of pressed fruit and middle-class aspiration, and it never disappoints. And every year, long after my son has moved on to the next obsession, I still pass a Christmas tree, think: “It’s not a fish!” and smile.