Going by what happens on chapter 23 he knew she had been held back once, so if I'm right she'd be a 3rd year if she hadn't been held back again.
He couldn't know whether she was attending again or not but there was a chance she was, or at least he probably thought so going by his reluctance to accept Hikari's invitation. Could be that there was other reason for not wanting to go but I doubt it.
We haven't really been given Yuu's POV (except briefly in the novelization of Chapter 38 at the end where he admits "I was the worst today, right?". So, since we haven't seen into his head we can only speculate as to why he never tried to get in touch with Yami after realizing he was blocked.
My current theory is his reluctance to go to the festival is two-fold. One - I think it's at this point in time he came up with a tentative plan to ask Hikari out at the festival but in typical Yuu-fashion he was hesitating/self-doubting himself and two - he doesn't want to run into Yami at all either because he's moved on or most likely because running into her would put a damper on his plans to confess to Hikari.
I feel Yuu's been wanting to man up and confess to Hikari as he kept getting hints from her that she liked him but, again in typical Japanese romance manga fashion, he has to have some sort of "perfect" moment which the festival would provide.
I'm going to go out on a limb and make a predication for next chapter. I think both Yami and Yuu have grown since their ill-fated romance. I believe they'll have a discussion where they air out what happened. Yami will apologize for ghosting Yuu and Yuu will say something like he enjoyed what they had but he's moved on. Yami will reveal she knows Hikari likes Yuu and vice versa and that she's best friends with Hikari. Yami might give Yuu a push to ask Hikari out or Yuu might tell Yami that he's planning to ask Hikari out. The kiss will not be a "I'm in love with you and want you back" thing. Instead it'll be a "we've both grown and learned something and this is a farewell/ending to what we had." However, if that is the case I think it's going to soar over the heads of a lot of people who come post in these threads lol.
