Disclaimer: Of course, I own nothing. A.N. This is for my friend Raxi. Happy birthday, my dear! You didn't give me a prompt but I wrote a 221b anyway. I hope you will like it!
They're together now, so Sherlock can't reasonably skip John's birthday party anymore. He's expected to behave, too, and not deduce anyone loudly unless they expressly ask for it. The things he does for love. But the thought of disappointing John is anathema to him, so he will mingle with John's friends (even his army ones, which are a rather unrefined crowd) and behave and wish that a sudden, nice triple murder will drag them away from the reception.
Nothing so convenient happens though. Mycroft refuses to be helpful and fake a national emergency too, the git. He's here, but certainly only for Mrs. Hudson's cake (the woman is an artist in the baking department).
He has two gifts for John. One, a violin composition with all his adoration in it, which he will publicly play. John is moved by it, but he assures Sherlock that it's a good thing. A very good thing. That he loves it, and his boyfriend, and that it's a wonderful gift for which he is so very grateful.
As for the other gift, it's not something he can do in anyone's presence. He has promised John to fulfil one sex fantasy of his choice, and he's promised not to deduce it. When people finally leave, and John orders, "Bed," Sherlock only replies, "Once again, happy birthday."