The last week has been very busy and difficult for you, without a minute of free time. Lots of homework, the end of the trimester, which you need to finish as well as possible, and the icing on the cake - exams that are about to come. There is no time at all for anything other than studying - not for yourself, not for anyone else.
Not that Denji is very indignant about this. Of course, he understands your workload, since he himself has encountered exactly the same problems as you. For him, the end of the trimester is still a novelty, because this is his first school year in all this time, and the guy has never encountered anything like this before. He has already dealt with the murder of demons, the death of loved ones, situations on the verge of life and death and all the other horrors of real life. But with evil teachers, bad grades and misunderstanding of the school curriculum - for the first time.
And at such a difficult time, Denji's birthday falls, which you, frankly, completely forgot about. You didn't even prepare some semblance of a gift, not to mention organizing a full-fledged celebration. Do not doubt it, you definitely look like a complete fool, standing in front of him empty-handed, having learned about his birthday from his lips.
"Eh.. Forgot? It's nothing. I didn't expect anything special, and I don't really want to celebrate.. So don't worry about it." The guy scratches his nape, looking at you. His words do not console you at all. You probably should have prepared a gift in advance or.. well, at least congratulated him verbally, and not let Denji brush off his holiday like that!
From Denji's stories, you knew that he had never celebrated his birthday normally - there was no way. Earlier, you were going to fix this, but you totally failed, and there is no excuse for you. But maybe there's still a chance to fix it?