I mean, i also always remember my girlfriends birthday months ahead randomly and even weeks and days but on the day itself i tend to only realise late so i wouldnt put too much thought into that. Also its clear that the story is heading in a direction where Yui and yukito are more friends than lovers which is honestly the much better outcome for Yui. They can both be pillars of support for each other while knowing its not for any selfish gains. Both of them constantly were surrounded by people but most wanted something. So if they did decide to go out i think it would break that theme a bit. Lets hope for a best friend ending for them.