@AokoPaizuri My best guess is it's from one of the KT side-stories, but I don't remember any of them mentioning Arcueid's birthday. They appear together in Plus Disc but that's in the cafe. Unless it's from a very early version of the scene in the mansion which didn't have this CG.
@Rand90017 I'm getting noticeably better at reading but I still end up procrastinating immersion for whatever reason. It doesn't make sense, because I always enjoy it when I actually do it but I have to force myself to get started.