For me, Christmas was always complicated. As a child, it was joyful, yet tinged with something bittersweet. On the one hand, there was my Scottish mother, who went all-out for tradition. She eagerly collected festive ornaments and planned menus from her beloved Supercook magazines. She would take us to the Walrus and the Carpenter, a traditional toy store on Sauchiehall Street in Glasgow, the memory of which still elicits wistful sighs from Glaswegians of a certain age.

On the other hand, there was my Bengali father. He was a reluctant participant in our Christmas extravaganza. It puzzled me that he couldn’t answer the most basic questions. Dasher, Dancer, Prancer … and then? He would smile sadly. “I’m not too sure.” How could anyone not know that? He seemed far more interested in an elephant called Ganesh who didn’t have to be put back in his box when the decorations came down.

His ambivalence intrigued me because it was so out of character. Normally, he quietly enjoyed life – food, wine, music and hospitality. To five-year-old me, the idea that someone might not love Christmas was radical and worth filing away for future reference. It was years before I realised that, even though he obeyed Mum’s orders to leave out Irn-Bru and carrots for the reindeer, my father’s own childhood had been a Santa-free zone. I was dimly aware of the Bengali equivalent to Christmas, Durga Puja, of course, which celebrates the victory of the Hindu goddess Durga over the demon king Mahishasura. Every September or October, airmail parcels would arrive, and ambitious plans were hatched to acquire syrupy cottage-cheese dumplings – called rasgulla – from Ambala Sweet Centre in London. But for me this was an addition to 25 December, not a substitution.

Matters were further complicated by my father’s job. After going to medical school in Kolkata, he had got a posting as a junior doctor at a hospital in Glasgow, where he met my mother, a nurse. He often had to work on Christmas Day, which would make Mum cross, although it wasn’t easy to take the moral high ground when competing with people who had acute liver failure. (She had given up work after having children.) My sister and I were largely oblivious to this tension, thrilled to go to work with Dad and see actual Santa visiting sick children on the wards.

As the years went by, though, my father began to embrace Christmas. He was promoted to consultant, so he didn’t have to work on the day. He would still mutter about the salmonella risk whenever Mum talked turkey. And he would skulk off to another room to listen to Miles Davis when the Christmas Eve carol service came on the telly. But he would happily return to pour the sherry and open presents with the rest of us at midnight. Everyone was joyful. Everyone except me, by then a teenager. Somehow, the more Dad engaged with Christmas, the more I morphed into the Grinch. It was as if festive malaise was one of the infectious diseases he specialised in – I had caught a terminal case, while he had gone on to make a full recovery.

Things were probably exacerbated in adulthood by the fact that not-so-fabulous events had a habit of happening to me at Christmas: losing a job, a brutal breakup, a health emergency. But as my droll mother once remarked: “You can’t blame Christmas for these things – it’s not Christmas’s fault.” It was as if Christmas was a person she had to loyally stick up for. My symptoms worsened – Christmas songs set my teeth on edge, and I’d long given up on sending cards or putting up decorations.

But then, one day everything changed. In October 2012, after a brief but complicated bout of pneumonia, my father died. Although he was 79, he had been in good health; nothing could have prepared us for such a loss. My mother was inconsolable. We all were. I have no recollection whatsoever of Christmas that year, except that it was the worst of my life.

In the months that followed, I fell into a state of limbo, as if one of the main characters had left the stage, and we were waiting for his return. Everything felt weirdly muted. Slowly, I drifted into that dark alley of loss where you feel guilty about enjoying yourself when the person you love is gone.

During that period, one of the only things that kept me sane was weekly choir practice. I had joined the year before Dad died and it had been a source of immense support while coping with grief. However, as anyone who has ever been in a choir can attest, Christmas is non-negotiable. A carol concert in Trafalgar Square was the last bloody thing I needed. But it was in aid of Shelter and the choir was short of altos.

So I dragged myself out on that freezing night. It’s hard to fully express what happened next, but I would describe it as an epiphany. I started off standing under that ridiculous 20-metre (65ft) Christmas tree, grumpily “ding-donging” through Carol of the Bells. I looked out across the square. The lights were sparkling; London had never looked so beautiful. I had totally lost my place on the sheet music, so I started dancing and ad-libbing. By the Jingle Bell Rock finale, I was laughing and doing jazz hands, carrying on like the naughty 15-year-old chorister I had been at school.

Anita Chaudhuri (foreground on the right) at the carol concert. Photograph: Courtesy of Anita Chaudhuri

Afterwards, everyone headed off to the pub, but I lingered, still mesmerised by the tree. It was then that a woman approached me. “Hi,” she faltered. “You were wonderful up there – I wanted to thank you.” I made a joke about how we didn’t sound as out of tune as normal, but she shook her head. “No, I wanted to thank you.” I couldn’t think what she meant. “Things aren’t so good for me at the moment,” she told me.

I scrutinised her more closely. The author Joan Didion wrote, in The Year of Magical Thinking, about how bereaved people can identify one another: “People who have recently lost someone have a certain look, recognisable maybe only to those who have seen that look on their own faces. I have noticed it on my face and I notice it now on others.”

Yet what I saw in the woman’s watery blue eyes was not altogether familiar. There was grief, yes, but there was a sense of something darker; a shipwreck. It dawned on me then that while I missed my father intensely, the loss had come after 48 years of his devoted attention. I didn’t need to hear this stranger’s story to intuit she hadn’t had that; what I’d had was a rare gift. Because of it, I would find my way back to myself; indeed, the process was already in motion. Not everyone was that lucky.

But the woman was still talking. “Seeing you up there having such a good time, it made me realise I’ve forgotten how to enjoy myself.” She made a show of jazz hands: “I’m going to remember to do this.”

Six years on, I am the one buying wrapping paper in July and making my own Christmas cards. It might sound strange, but that generous conversation somehow gave me permission to get back to the serious business of enjoying life – and Christmas. Not only was it what my father would have wanted, but doing so could have a positive impact on others – even perfect strangers.