It's probably about how important anniversaries are to them! Hyunsoo doesn't seem to get how important it is to Youngchan. And Youngchan doesn't communicate and spirals in his own head.
But there's a chance they have different dates as their anniversary date. And also he could've just forgotten or maybe he do have something planned( doesn't look like it rn tho), he might've noticed it but probably don't think people take it that seriously looking at his personality.
I'm curious too tho, what you said does make sense
But it's not the anniversary that Hyunsoo is busy during but the evening before. That's definitely something Youngchan should've clarified cuz Hyundoo was probably just focusing on the day of, not a day in advance.
Ikr!!!
Idk it just doesn’t seem like him to forget dates and anniversaries to things (I mean wouldn’t he write it down somewhere too? Hes kinda particular like that imo) so I’m wondering What the actual miscomm here is