In retrospect, Stiles thinks that he really should've known better.
Lydia and Jackson breaking up isn't anything new. It happens at least once a month and usually only lasts for a week. They're one of the school's hottest couples, but they're still a lot better at fighting than they are at communicating, or generally getting along. (Stiles has several theories on why this is, any one of them could be true.)
Stiles hears about this most recent breakup on a Wednesday, he somehow gets involved in the drama the following Thursday. It's not a particularly interesting Thursday. It's a Thursday like any other Thursday. Stiles got up that morning, drove him and Scott to school, and then suffered through a few classes in which he was mostly bored. Lunch is when everything changes.
Stiles doesn't go to the cafeteria for lunch on that particular Thursday. Scott had forgotten to do his English essay the night before and Stiles had agreed to help him finish it over lunch. Stiles is heading to the library when he comes across Lydia. Or, she comes across him. He's all set to stand aside and have his compliments and greetings be ignored, as per usual, but his "You look lovely today, Lydia" dies a swift death halfway out of his mouth when Lydia actually acknowledges his existence and makes a beeline right for him.
Her eyes are wide and sparkling, her red hair is shining from the sunlight coming through a nearby window, and she's smiling widely at him. Thinking back on things later, Stiles knows that this should have been his first clue. He's always had the hope that, eventually, his persistence would pay off and Lydia would have to develop feelings for him. At this point, though, he's well aware that she isn't overly fond of him (when she even knows he's alive, anyway).
"Stiles!" Lydia exclaims, that Thursday afternoon. "Just the person I was hoping to find today!"
"I—wha—really?" Stiles asks, pointing at himself like a dork. "I—okay, here I am!" He spreads his arms wide in an exaggerated motion and nearly knocks something from a nearby bulletin board. Lydia briefly glances over at it, and then back to him, her smile never wavers. That right there should've had alarm bells going off in Stiles' head, but he had been too preoccupied with OMG Lydia Martin is talking to ME AND SMILING!!11!!1!!1
"How would you," Lydia starts, immediately closing the distance between her and Stiles. She rests a hand on Stiles' chest and Stiles sincerely hopes she can't feel how hard and fast his heart is pounding. "Like to take me out tomorrow night?"
"Like, on a date?" Stiles asks.
"Exactly like on a date," Lydia says, still smiling. Stiles is nodding like a bobblehead before his mouth decides to answer.
"Yes!" He exclaims. "Totally. I would love to take you out on a date tomorrow night."
"Excellent," Lydia says. "I'll pick you up at 7. I'm not getting near that death trap you call a Jeep." Stiles just nods dumbly. Lydia pats him on the chest and saunters off down the hallway. Stiles is completely frozen as he watches her go, only jumping into motion when Lydia reaches the end of the hall and disappears around the corner.
"Oh my god," he mumbles. "Scott. SCOTT!"
Scott had initially been skeptical when Stiles told him about the date, but eventually his eternal sense of optimism had won out and he'd been nearly just as excited.
Stiles doesn't know what his father's reaction would be to the whole situation, and he never really finds out beforehand because the sheriff is working the evening and night shift all week and Stiles never gets a chance to tell him.
Scott and Stiles text back and forth continuously Friday night while Stiles waits for Lydia to pick him up. She shows up at 7 o'clock on the dot and Scott's final text is a 'good luck dude' as Stiles climbs into Lydia's car.
He doesn't question why they drive to a restaurant in the next town for a meal, and doesn't disagree when Lydia tells him that they should take this to a more private venue. And he definitely has no issues when Lydia ends up driving them up to her family's secluded lake house in the woods.
Stiles genuinely wasn't expecting things to go as far as they did, but he can't say he's anything but happy when he's lying, sweaty and sated, next to Lydia in one of the lake house's bedrooms. He really hopes the experience was as good for her as it was for him, but they don't end up lingering around long enough for Stiles to ask.
He's a bit disappointed that there isn't any post-coital cuddling, but the smile Lydia gives him when she drops him off home gives him a bit of hope that maybe, just maybe, this could be the start of something great.