It was only when I was posting a card for my old friend Stamford that it occurred to me how few birthdays people seemed to celebrate nowadays. When I'd been at medical school and then in work, there had always been people in my dormitory, my friendship group, my office who had brought in cake, or had little parties down the pub – that sort of thing. Even in the army my comrades would receive cards and bars of chocolate through the post from assorted friends, and we'd all celebrate together if we could. But now my life seemed almost free of the childish, slightly cheesy parties that had filled it before, probably due to the fact that I lived with Sherlock Holmes and Sherlock didn't do birthdays.

Well, we'd been in Baker Street almost a year and he was certain to have had a birthday in that time, or to have one coming up; but he hadn't written it in the calendar or mentioned it, and when I questioned him gave me one of those enigmatic glances that means he doesn't want to reply. Evidently he wanted to avoid all that ridiculous stuff – the "bumps", out-of-tune singing, bizarre and sometimes inappropriate presents, cards with your age on them (they're terrifying). I mean, most people want to get away from that stuff when it's happening to them, but it's only a spoilsport who actually avoids it. I'd had a birthday; Sherlock had got me a new coat that was a bit like his – actually, more than a bit like his. ("You have no taste in coats, John. That isn't a coat, it's a jacket." Scathing glance in the direction of the coat-hooks.) Sometimes I wear it and people mistake me for him. But anyway, the point I'm getting at is that I didn't know when his birthday was and I felt bad because I might have missed it, and he wouldn't have said anything even if it had mattered to him, so I was determined to find out.

Sherlock knew exactly when my birthday was. I don't know how. A week beforehand he said, 'I suppose you'll be out Monday evening?' and I had to nod, because I was going for a pint with Stamford and a bit of a catch-up. Not much of a party, but it was quite a nice evening. I never asked how he had guessed or found out, because that would lead to a fifty-miles-per-hour explanation of his deductions and me feeling even more confused than before.

Sadly, I couldn't pull off the same trick. I scrutinised the calendar for even the slightest of marks; I asked Sherlock again, to no avail; I even glanced through his phone, but that has almost nothing on it. In the end I decided on a course of action: to phone Mycroft. He was about the only one who would know.

'Mycroft, when's Sherlock's birthday?'

'His birthday? Hasn't he told you? – Oh, but of course. My dear brother hasn't liked birthdays since...' He hesitated. 'Well, since he was born, to be perfectly frank.'

'But when is it?'

'The 6th of January. But be careful.'

I could almost hear the raised eyebrow, the slight smirk that accompanied Mycroft's enigmatic warning. Then he hung up and I smiled vaguely, wondering what to do with the information now I had got it.

A week later, on the 6th of January, all was prepared. Just as I was adding the final touches to what was, quite frankly, a masterpiece, I had a sudden thought: what if Sherlock hated me for this? What if he completely scorned it, and would never trust me again? It was a pretty random thought, and probably quite extreme, but you never know with Sherlock. He's a quirky and unpredictable character, and I still wonder sometimes if I know him well enough to count him among my definite friends; but eventually I shook my fears aside and, after having switched the light off, I took my place along with the others.

We all heard the door to 221 Baker Street open with a creak, a light footstep on the welcome mat (which, to be honest, lies most of the time), and held our breaths. The apprehension was building. We listened as Sherlock ascended the stairs, carefully and measuredly, as usual, and came to the door to our rooms. A moment's hesitation. Then he opened it, stepped into the darkness, didn't think to turn on the light, merely stopping in the doorway, silhouetted against the light coming in from the hallway.

'Well, that was a splendid surprise. Magnificently done. Hullo everyone.'

His voice dripped with characteristic sarcasm, and everyone in the room – Lestrade, Molly, Mrs Hudson, Sarah, Mycroft (that had taken a lot of persuading, but he had come along to "see the fireworks") and me – let out a groan. But of course. He had probably guessed my plan from the moment at which I had conceived it in my mind. Sherlock went to the light switch and flicked it casually, illuminating the small stack of presents and cards on the coffee-table and the spread on the kitchen table, of which the centrepiece was a rather marvellous cake, ordered from the bakery that very afternoon by our long-suffering Mrs Hudson.

He looked around at everything. The envelopes addressed to him. The slightly worried faces watching him. The inscription on the cake – "Happy Birthday, Sherlock".

'Mycroft told you when my birthday was...' Sherlock was talking to me but looking accusingly at Mycroft.

'That's your present from me,' replied Mycroft, deadpan.

'Thank you, dear brother.'

The atmosphere in the room was crackling like an approaching thunderstorm. Mrs Hudson stepped in to prevent the fireworks that Mycroft had hoped for. 'Sherlock dear, why don't we all have tea whilst everything's fresh?'

Sherlock gave a curt nod and we all moved towards the table, taking our places with measured silence, still watching Sherlock. He sat at the head and looked witheringly towards the paper hat (stolen from a spare Christmas cracker, admittedly) that we had put on his plate, before brushing it aside and pausing a moment.

'You don't have to wait for me to start,' he said quietly.

We took this as the signal to begin. Mrs Hudson poured out wine into glasses I hadn't known we had until that evening, and distributed them among the awkward guests to try to make the party a bit merrier (so to speak). Sherlock sipped from his and ate food in tiny bites, not consuming more than a few mouthfuls during the entire meal; but the rest of us allowed ourselves a minor indulgence, there being good food and plenty of it. It was a bonus that none of it had shared a fridge with a body part, like most of the things I ate.

Stilted conversation began a minute into the meal. Everyone inquired how everyone else was, despite us having been together for most of the afternoon; we wished Sherlock so many happy returns that we began to fear that we would make him immortal. He took this all in his stride, hiding any emotions he might have felt behind a somewhat shadowed, blank face. But it is hard to guard one's eyes from feeling, and now his were twinkling mysteriously with something I could not place.

Once the dinner was finished Mrs Hudson brought out some candles and sunk them carefully into the icing on the cake.

'Oh no, not candles,' groaned Sherlock; this was perhaps his longest sentence since before tea.

'Yes, Sherlock, candles,' Mrs Hudson said in her exasperated mother voice, whilst behind her Mycroft smirked slightly.

'And you have to make a wish,' I cut in, smiling myself now. Sherlock wasn't completely annoyed; I could see that he was keeping up that appearance to preserve his pride.

Mrs Hudson lit the candles and motioned to Molly to put out the lights. It was a bizarre sight when the room fell into darkness once again, of Sherlock's face floodlit by the orange warmth, his cheekbones exaggerated by the shadows that fell beneath them, almost the only thing visible except for the blue shine of his eyes. These suddenly betrayed a sort of panic: it was time to sing.

'Happy birthday to you, happy birthday to you, happy birthday, dear Sherlock, happy birthday to you...'

The strange cacophony, consisting of the terrible singing of me and Lestrade, the trying-desperately-to-hit-the-notes efforts by Molly and Sarah, and the surprisingly good tenor voice of Mycroft at last came to an end, and we all picked up our cutlery or balled up our fists and banged on the table, as if we were schoolchildren once again. And for a second – if I had blinked I would have missed it – a smile came to Sherlock's face at last, almost obscured by the darkness, but outlined in the dim light from the candles. Some memory traversed his mind; there was a glow of something in his eyes – and then his neutral expression returned, and with more than a slight reluctance he blew out the candles. As the smoke drifted away I wondered if he had made a wish, or whether he had dismissed it as silly, along with everything else associated with birthdays.

Molly jumped up and put the light back on; and Sherlock cut the cake, handing out slices in silence, looking rather pensive – indeed lost in his thoughts. I found myself trying to guess what he was thinking about. Doubtless he, in my position, would have known instantly.

The party didn't last much longer after that. Sherlock, growing tired (or saying so – he doesn't like late nights, as they "disturb his mind" by disrupting his routine), rather impolitely asked that everyone begin to leave before he had even touched his presents – he would open them tomorrow. And he had already guessed what they were anyway. So the guests began to depart, chattering amongst themselves, thanking me for organising the party (never once saying that it had been something of a fiasco, but implying that) and wishing Sherlock a happy birthday, knowing, of course, that they would just get a stony stare in reply.

And when they had all left Sherlock made to go to bed but I stopped him.

'You can say that you hated it, if you like.'

He raised one eyebrow. 'I could – but it wouldn't be entirely true.'

Our eyes met. I swallowed, wondering whether he would elaborate. He didn't.

'Did you make a wish?' I asked casually, as I was still curious.

He did not blink. 'Yes.'

'Oh, really? I didn't realise you were superstitious.'

'I'm not; I just wished that –

'You don't have to tell me,' I said, and smiled. 'It wouldn't come true then.'

'That is just superstition.' He seemed to argue with himself before saying, 'I wished that I would always have such good... good acquaintances.'

He stopped a moment, as if regretting speaking, but he did not finish there.

'And I wished that I would never lose my only friend.'

A second – a century! – passed. His piercing eyes burned into me, and I saw to my astonishment that the corners of them were sparkling. Then he went from the room, leaving me to dwell on the evening's events.