Based on the headcannon by Fembot77: Beckett gets a delivery of 8 roses, with a note "A rose for every year since we first met. Happy Anniversary"
A few months later, on a Wednesday afternoon, Castle opens the door to a flower delivery with his original note attached. There are 17 roses.

Writing Roses

Kate ran her fingers through her sleep-mussed hair while she paced her apartment. Her cousin Sophia's apartment. It was stupid. So stupid. Not something she should be so worried about. It was just a day like any other. But it wasn't.

It wasn't like she and Castle celebrated their "workiversary". Nobody did. Except Ryan and Esposito but they were bromantic like that. But she and Castle? No. Never. Well, he had taken to bringing her extra special coffees on March 9 of every year. And there was that one time he bought out the pastry shop she loved. And the year before there was a balloon. One. Singular. But it wasn't like they celebrated, celebrated.

But this year was different. This year they were "separated" and Castle wasn't sitting in that stained brown-green chair next to her desk. He wasn't taunting her into betting how many M&Ms he could catch in his mouth, and she wasn't feigning exasperation every time one smacked him in the eye. This year they weren't together, strolling down the sidewalk to the precinct with matching travel mugs clutched in their hands while they searched for the first vestiges of spring. This year she was pacing her apartment– her old apartment, alone, like it was eight years and one day ago– and Castle was in L.A. without her.

Kate sighed when her eyes landed on the clock. Nine AM on her day off. She was supposed to have met Vikram at the club half an hour ago. He had already texted twice. She stared at the messages glowing on the lock screen, fingers tangled in her hair, digging at her scalp.

Exhaustion flooded her system, shoulders sagging under its weight. Her gaze wandered from the phone to the bed. The pile of rumpled blanket called to her. She never slept well alone anymore.

The shower had helped– scalding water burning away fitful sleep full of cryptic dreams. The eyeliner and mascara helped Atlas pull the weight from her shoulders. The high ponytail and higher heels reaffirmed her confidence that she could make it through this day. Alone.

She almost tripped over them outside her door. Eight single stem roses nestled in a vase on the cheerful welcome mat Sophia had purchased before leaving for Europe. Kate crouched down in the open doorway, a small smile painting her lips when she ran her finger over the ridge of a bright red petal. She picked them up, careful not to jostle them when she carried them inside, placed them on the counter, before drawing the card out from between thorny stems.

"A rose for every year since we first met. Happy Anniversary."

The smile on her face blossomed until it crinkled at her eyes.

She could make it through this day, because she wasn't alone.

The knock on the door came at 10:27 in the morning. Castle stumbled out of his room, one hand adjusting the hem of his t-shirt while the other rubbed sleep from his eye.

Everything was back to normal. Kate was home. LockSat was no more. Hayley and Alexis had the PI office under control. High Heat was set to hit shelves later in the summer. Yet he was at home, alone, while Beckett had a morning full of dull meetings and bureaucratic nonsense.

Politics and paperwork.

A smile tugged at his lips just like it had a couple months before when his Beckett had echoed the same sentiment as Alt-Beckett.

Maybe he would get lucky and his phone would ping with an "I'm so bored, entertain me so I don't fall asleep!" text from his wife.

The fist pounded on the door again and he picked up his shuffle, huffing for them to "Hold on. Just a second" under his breath.

The courier shoved the vase full of flowers in his face before Castle could mumble a greeting. And he yelled a thank you, when the kid scuttled off down the hall, one hand hiking up his pants as he went.

Castle inspected the bouquet of long stemmed red roses with a frown. The card had his name on it. Last time he checked he didn't have a secret admirer or stalker. Though that would be kind of cool… creepy but cool.

He set them down on the kitchen island and flipped the switch on the coffee pot before returning to pluck the card from the holder. His frown deepened when his own words, his own handwriting, stared back at him.

"A rose for every year since we first met. Happy Anniversary."

Rick's eyes flitted from the card to the roses and back again. Every year… But there were more than eight roses in the vase. His fingers made quick work of the count, tapping each rose on the head before doubling back and counting them again.

Seventeen.

But…

"You know, you were right. We have met before… On a book tour for an old Derrick Storm novel. I, uh, waited in line to get it signed. By you. I used to love those books."

No…

The words echoed in Castle's brain as he skittered around the island, dodging furniture while he ran for their office.

Seventeen years. Nine years before Allison Tinsdale. June. 1999.

Castle stopped in his tracks, his breath knocked loose from his lungs.

1999. Right after…

He shook his head, it was not the time to dwell on that. Not the time to think about how hollow her eyes must have been, to wonder if his heart had ached for her then the way it did now.

He scootched her desk out of the way, careful not to disturb her computer, and knelt down to examine her collection of his novels.

Did she smile? Did her lips quiver in a failed attempt?

June 1999 that would have been… a ha! His fingers plucked the second Derek Storm novel off the shelf and he flipped open the cover with bated breath.

"Kate. Thank you for being a fan. Richard Castle. June 6, 1999."

His heart plummeted to his gut. His words, his hand writing. So generic.

His younger self was an idiot.

With a sigh Castle pushed himself to his feet, knees and back cracking with the effort.

He settled at his desk and picked a felt tip pen from the holder. Opening the book in front of him, he wrote.

Five minutes later he slid the book back into its spot on the shelf for Kate to find later. After a shower and with clean clothes, he set about crafting one cappuccino and one perfect steamed latte with two pumps of sugar-free vanilla syrup.

They had an anniversary to celebrate.

A/N: for Dia.

Thank you to Molly (Muppet47) for the beta. :)