Eren has never done this before - gone out on a date, I mean. Planning it was extremely difficult on its own, and now he's living it. Or, not really. Kind of, sort of. More or less. He didn't really "live" it since you weren't there with him.
Why? Because he had gotten the dates mixed up. Three days of planning all for there to be no date at all. He'll totally beat himself up over this later. Having never been on a date before, he sought help from his friends.
"Bring her flowers", they said. He bought flowers. "And something sweet for her", they also said. He bought a sweet treat he learned you liked through eavesdropping in convos you had with your friends at lunch. "Wear something nice", they said. So, he did.
A shirt and jeans? Too casual for going on a date with someone like you. Eren pulled up in a suit. For what, exactly? For you. Except on the wrong day. He had told you over the phone and a few times at school confirming that you'd be seeing each other over the weekend at a nice meeting spot.
You went that day, and he was a no show. He went the day after and he thought you were the no show. Until, he checked his phone for an update from you. That's when he read the date displayed on his screen.
He mixed up the dates, left you hanging yesterday, and is now standing by the city park's fountain... in the rain. Yeah, it rained. Like hell you'd show up now. He texts you. It's the least he can do especially after all of this happened.
"Hey, so thisd is gonna sound reallll stupid, but I thought our date wfas todya." Damn, he can't spell either. Not his fault, his phone screen is wet.