"No, don't!"
Holmes' exclamation was so explosive Watson stopped immediately and spun around to face him.
"Holmes? What's wrong?" he asked.
"It's Christmastime, Watson!"
Watson blinked at him in confusion. "Yes?"
"Yes! So leave it alone!"
Watson looked in between Holmes and the spider.
"Peace on earth, goodwill towards insects?" he asked with a smirk, but didn't lower his rolled-up newspaper.
"It's not an insect, it's an arachnid," Holmes sniffed, "and you can't kill it at Christmastime."
"I fail to see why not."
Holmes stepped over to him, huffing in annoyance and pushing the newspaper away. He picked up the spider easily, gently cradling it in his palm and covering it with his other hand. "I'm surprised at you, Watson," he murmured. "I wouldn't think you, of all people, would kill a spider at Christmas."
"You still haven't told me why I shouldn't," Watson protested.
Holmes gently laid the spider on the branch of their Christmas tree. "Really, Watson, you're the literary man among us. You mean to say you really don't know? Even Mrs. Hudson knows, and she's never celebrated Christmas anywhere outside of the British Isles."
Watson grinned very slightly. "Tell me the story," he said.
Holmes watched the spider scurry into the tree. "A long time ago," he murmured, "a kind widow was too poor to decorate a Christmas tree for her children. So, on Christmas Eve, the spiders who lived in her home came out of hiding to weave their webs over the tree for her. In the morning, she was frightened by the spider webs and so she threw her curtains open. When the sunlight hit the webs, they turned to silver and gold. From then on, the widow prospered and her children never went without. That's why you must always be kind to spiders, especially at Christmastime."
"I don't think we will get silver and gold from this little one," Watson pointed out, but there was no malice in his voice.
"Perhaps, perhaps not. But look, Watson, he is already decorating our tree."
Watson couldn't help it, he grimaced as he saw the thing spinning a web through the branches. "And if it's not a little man and is instead a little lady who will shortly fill our rooms with her babies?"
"She won't," Holmes said. "You are quite safe from an infestation."
"How can you tell?"
"It is Christmas, and it likes us."
Watson snorted.
"Besides," Holmes continued, "as a medical man I'm sure you're well aware that we need as many spiders in the world as we can get. After all, they save as many lives as you do as a doctor, not by treatment but by prevention. Perhaps this one here has already trapped and killed an insect in this house which would have otherwise bitten one of us and caused us ill at Christmastime. Is that what you want? To be ill on Christmas Day?"
"That's not very likely, Holmes."
"Yet who can say, Watson? I suppose…" he sighed. "Do you really want it gone?"
"No, Holmes," Watson said with a smile. "I admit being nice to spiders has never been a general policy of mine, much less a tradition, and, frankly, I think it is a bit ridiculous, but of course I will respect your wishes. After all, I am a sap for a good story, especially at Christmas. You must agree to something, though, my friend."
"What is it?"
"If it crawls out from behind my inkwell again while I am concentrating, then it's got to go."
"It won't," Holmes said seriously. "It knows it's Christmas." He kept a straight face for as long as he could, but finally broke when Watson rolled his eyes dramatically at him.
"I like spiders," he grumbled.
"And I hate them!" Watson laughed. "But you're quite right: it's Christmas! I'm quite willing to believe even spiders are celebrating. You're in charge of that one, though, and don't let Mrs. Hudson see it." Watson patted him on the arm before turning back to his writing.
It was a couple days later on Christmas morning that Watson saw a new decoration on the tree: a little golden spider with diamonds for eyes hanging from a long, delicate golden chain. He laughed, plucking it off the tree and holding it up to the sunlight. Holmes was on the other side of the tree, pulling his gift out of its branches.
"What a shame," he murmured as he did so. "Mrs. Hudson found our little friend the spider and cleared his web away. I was going to move him out today."
"You had a spider in the tree?" demanded Mrs. Hudson, coming into the room with a tray of coffee. Holmes stepped away, his eyebrows knitting. He looked at Mrs. Hudson, who looked at Watson, who looked at Holmes. And then, slowly, all their heads turned to the little golden spider Watson was holding up in the sunlight.
After a long minute of silence, Holmes finally moved. He crossed to Watson and clapped his friend on the back. "I suppose… Merry Christmas, Watson," he said, and his voice raised a bit a the end as if he was asking a question.
Watson nodded, and didn't ask if Holmes had bought the decoration. Holmes, for his part, never said anything more about it; whether to keep the mystery or because he'd rather not discuss it out of fear of the conclusion he'd come to, Watson never learned. Watson suspected he knew, however, and from then on until both the men had moved out, the spider hung every Christmas on their tree, and nevermore was an arachnid killed in Baker Street.
For the prompt from Michael JG Meathook: The strangest Christmas tradition in the world.
This might not be the strangest Christmas tradition in the world, but to the author, who is much more like Watson than Holmes when it comes to spiders, it is certainly near the top of the list.