This is the first stab at this concept, and I’m not sure how well it turned out. It’s probably something I’m going to revisit — probably even this very scenario. We’ll see. For now see what you think!

Julie opened her eyes and squinted over the blanket lump that was her date from last night at her alarm clock to see it was only 7am on New Years Day. She did the mental calculations and realized two facts at once: 1) she’d only gotten four hours of sleep, and 2) she wasn’t going to go back to sleep. Shortly after that, another fact jolted through her: Dave and I slept together! she thought, eyeing the brown head of hair peeking out of the covers, snoring quietly next to her. The jolt subsided and she thought with more than a little resignation, Yep, Dave and I slept together…
They’d known each other forever, and her crush on him had waxed and waned over the years — as, he admitted last night, his for her had as well — but the timing was never right: he was in a relationship, or she was dating someone who seemed right at the time, or… so many “or”s. But last night… things clicked — at least they did logistically. As for the actual business… it left a lot to be desired. Julie sighed and Dave stirred at the sound. He rolled over, blinking the sleep out of his eyes.
“Wait…” he said as recognition settled in, “Did we…?”
“Uh huh.”
“We did, didn’t we?”
“Yep.”
A heavy silence stretched for long seconds. “…It wasn’t very good, was it?” Dave said finally.
“No, no it wasn’t.”
“GOD FUCKING DAMNIT!” Dave yelled, leaping out of bed.
“Whoa, whoa, whoa,” Julie tried to settle him down. “It wasn’t that bad,” though the more she thought about it…
“This isn’t how it’s supposed to be!”
Julie shrugged, “Dave… it is what it is. It’s not worth getting worked up about.
“No, no, no,” he said gesticulating wildly — which made more than just his finger wag. “You don’t understand, I lived December 31st, 2017 six hundred and thirty six times!”
“What do you mean? Lived six hundred thirty six—”
“Six hundred thirty seven including last night! But that’s the thing! ‘Last night’! There was a ‘last night’!”
“Of course there was. Dave, are you feeling okay?”
“No! I’m decidedly not okay!”
“Fine, do you at least want to put some clothes on?”
He looked down and realized he was naked for the first time since waking up. “Oh, uh, yeah…” As he searched around the bedroom for his boxer shorts he started to explain, “You know that movie ‘Groundhog Day’?”
“Of course.”
“That was yesterday!!”
“What do you mean? We didn’t watch that yesterday…”
“No, I was Groundhog Day-ed!”
“That’s crazy, Dave. Look,” she tried to console, “we just…. you know, maybe we’re just not meant…”
“No! I spent six hundred and thirty six days trying to figure it out. It took me forever just to figure out that I was supposed to get together with you…”
“Me? Why me? Did you get some sort of rule book?”
“What? No. I mean… You’re my Andie MacDowell!”
“Uh… Thanks?”
“It made sense! I mean, it always seemed like there could have been a thing between us if the timing was right, and I told you that last night and… oh no,” his face went pale and he sat heavily on the foot of the bed.
“Dave? What?”
“Was I a pity fuck?”
“Dave…”
“No, really, did you feel sorry for me when I told you I’d always had a crush on you took pity on me and…”
“Dave, seriously, it wasn’t….”
“Oh my God. Oh my God…” he started repeating.
“Dave! Stop. Don’t be an idiot. It wasn’t like that!”
“Really?” he asked sheepishly.
“Really. Look, we tried it. It wasn’t great. Live and learn, right?”
Dave sighed heavily, “Now what?”
She smiled, leaned forward holding the blankets over her chest, and kissed his forehead. “Let’s go have some breakfast.”
Dave stood and gathered his clothes and moved toward the bathroom. Julie took a look at her crumpled dress from the night before and shook her head at the memory. She pulled on a pair of sweat pants and a tshirt and went to the kitchen to make coffee.
Then she screamed.
Dave came running out of the bathroom, one leg in his pants, one leg out. “What?!” he yelled. He saw her standing in the hallway, then, past her, he saw a man in a suit pouring a cup of coffee. “Who the hell are you?”
The man looked up from pouring the coffee. “Hi Dave, Julie,” he started, then gestured towards the coffee pot, “I made enough for both of you.”
“Who are you and why are you in my house? And… and…. you made coffee?!”
“There’s bacon in the oven — should be ready in about five minutes. Oh! Muffins, too,” he said, opening the microwave and retrieving a plate full of blueberry muffins.”
“WHY ARE YOU IN MY HOUSE?!” Julie yelled.
“Ah, that…” he said, straightening. Then, in a more serious tone he said, “We have to have a talk. You’d probably better sit down…” he gestured towards the table.
“WHY ARE YOU—”
“In your house, yes yes. I’ll explain. Seriously, sit down first.”
Dave and Julie cautiously moved towards the table neither taking their eyes off of the man, like he was some sort of wild cat that would pounce any moment. They sat down and the man set a plate with muffins, fruit salad, and bacon in front of them along with a mug of coffee.
“Julie, a little sugar and Half and Half? Just like you like it, right?”
“How did you…? I didn’t even have Half and Half…”
“Right… that’s not terribly important,” he waved his hand. “I’m… well, you can just call me Bob.”
“Bob?” Julie asked.
“Okay, look… I’m not good with names and maybe that’s a bad one. Let’s just go with that for now. Let’s talk about last night…”
“Last night?” Julie asked.
“Yes, specifically you two,” he gestured between Dave and Julie. “You know, when you two ‘hooked up’?”
“How do you know…”
Bob held up a hand interrupting. “Again, not important. What is important is that it happened and, Dave, I think you’ve noticed… it’s no longer yesterday.”
Dave’s eyes went wide, “What do you know about that?!”
“Let’s just say this,” he gestured to the breakfast, “is about that. Eat! Please!” Bob was quiet until both Dave and Julie nibbled experimentally at the food in front of them. “Now then, Dave, you probably noticed that last night… wasn’t what you might have expected.”
“Yeah,” Dave said, his shoulders sagging.
“Whoa, Romeo,” Julie said, “It wasn’t a picnic for me either!”
Bob let out a laugh, “Right. So, here’s where it gets interesting — seriously, those blueberry muffins are delectable. Fresh blueberries!” he started unwrapping a muffin, then stopped. “Dave? Did you explain the whole six hundred thirty seven New Years Eves?”
“I tried…”
“Okay,” Bob said taking a bite of the muffin. He chewed thoughtfully and swallowed. “Dave, you’re right now living day six hundred thirty eight. I must say, I think last night’s performance got you to — you’re more upset yesterday than you are today!”
“Wait… upset yesterday…?” Julie asked.
“Are you familiar with the Many-worlds interpretation? The idea that when you make a decision you unknowingly create a parallel universe where you made the opposite decision?”
Dave shook his head. “I’ve heard of that,” Julie said.
“Good, we’ve got one smart one in the room,” Bob remarked snarkily. “Welcome to the parallel universe!” Bob said, spreading his arms wide.
“Huh?” both Dave and Julie said in unison.
“Dave didn’t really wake up today…”
“I did!”
“No, you did,” he gestured to Dave, “but the you-you is still repeating New Years Eve. You’re a parallel universe.”
“It just feels like the regular universe,” Julie said shaking her head.
“Right? Well, it does for now. But that’s why I’m here…”
“Why are you here?” Julie asked.
“Because this reality, by definition isn’t stable. It’s going to fall apart. I just wanted to explain this and, you know, make you breakfast before the end of your world.”
“Uh, that’s nice of you and everything, but… end of the world?” Julie said, then noticed the room get darker as if a cloud passed in front of the sun.
“Yeah, looks like it’s starting sooner than I thought,” Bob said frowning. “Well, eat up!” he gestured to the food.