Author's Note: Written for unbidden_truth for fandom_stocking.
Spock grips the package in his hands a little tighter as he makes his way to Kirk's quarters. This is not a human ritual he has much experience with, and he is uncertain as to whether it is truly the best course of action. He hopes that Kirk will enjoy his gift in the spirit it was intended, but he does not wish to inadvertently upset him or bring up bad memories.
By the time he reaches Kirk's door, Spock has nearly talked himself out of his decision. He stands there for a long moment, staring at the closed door, before giving himself a mental shake and buzzing for entry. He has come this far, he may as well complete the process.
The door slides open and Spock steps inside. Kirk is sitting at his desk, working, but he looks up expectantly as Spock approaches.
Unable to find the correct words for the situation, Spock instead settles for laying the paper-wrapped package on Kirk's desk.
Kirk stares at it. "Is that for me?"
Spock nods, folding his hands behind his back. "For your birthday."
Kirk's expression softens into a smile. "Wow, thanks, Spock. That's really nice. But, uh, today's not my birthday."
Spock frowns. "I consulted the data banks…"
Kirk shakes his head. "No, I mean, yeah, today's my actual birth date, but it's not the day I celebrate it."
Spock tilts his head, confused. "I am not sure I understand."
Kirk leans back in his seat, running a hand through his hair. "Today was never my birthday; it was the day my dad died. Mom realised that after she spent pretty much my entire first birthday party in tears, and she decided that I should have a birthday that was really about me and not him." He shrugs. "She picked March 12th, which is when the doctors said I should have been born, and so that's when we celebrated it. Ever since I can remember."
"I see," Spock says. "So what you are saying is that I should take my gift back and keep it until then." He goes to pick up the package, but Kirk gets there first, grabbing it and holding it to him protectively.
"I didn't say that," he says. "Now that it's here it would be rude not to open it. Especially since you went to so much trouble and all." He sets the gift down and begins pulling off the paper.
Spock holds his breath as Kirk uncovers the scale model of the Enterprise, but he needn't have worried. Kirk's face lights up, fingers carefully exploring the nacelles, the saucer, the painted letters on the hull. "It is a perfect replica," Spock tells him.
Kirk glances up at him with a smile. "It's awesome," he says, reaching out to wrap his fingers around Spock's arm. "Thank you."
"You are welcome," Spock replies. His surprise may not have gone quite according to plan, but as he looks at Kirk's bright smile, it seems to have had the desired effect. "Happy not-birthday."
"Yes," Kirk says, still smiling. "I think it is."
Note: Canon states TOS Kirk was actually born on the 22nd of March, but I figure he was a bit late.