Usopp had never really experienced Valentine's Day. He grew up, for the most part, alone, and boys don't usually give valentines to other boys. Kaya and Usopp always exchanged cards, and Usopp only mustered up the courage to give her chocolates when she started getting sicker. It was another way to cheer her up.
But since he was a kid, Kaya was like his sister, he always knew she'd make him a card because that was what they did, it had become an annual exchange that they did, and it was expected to get at least the one from her.
On the Going Merry, Valentine's Day wasn't special for anyone except Sanji, who thought it was an absolutely romantic holiday. His whole day was spent catering to Nami's ever whim even more so than usual, and he only stopped when she told him she had had enough.
Luffy had caught on soon after, and decided that since he loved all of his friends equally, he'd give everyone a valentine. This usually meant a sheet of newspaper with a large red sloppily drawn heart scrawled across the top. The "card" alone was an overall appalling gesture, but everyone knew what Luffy's intentions were, and were more than happy to receive anything he felt like making that day.
Usopp always forgot the day had arrived until Luffy's newspaper heart was shoved into his face. A lot of the time, he didn't pay attention to time at all.
Sanji emerged from the galley, a small plate in hand, a heart shaped card in the other, and sauntered down the steps to the deck. Usopp expected that he was giving Nami another valentine, probably the fortieth she'd received that day, and so he dismissed Sanji completely and went back to watching the sunset. He was surprised when Sanji's shoes stopped next to him.
"This is for you." Sanji didn't sound angry or irritated, he didn't sound like he was forcing this in any way. "Happy Valentine's Day." He bent down beside the sniper, and put a strawberry tart and the card in Usopp's lap, leaving him with a kiss on the cheek. Too shocked to respond, Usopp sputters and stares at Sanji's back as he walks away.
Sanji was the reason that Valentine's Day would always be special for Usopp.