This is the twoshot that convinced me that short manga can also tell a beautiful story. I downloaded it off DokiDoki about a year after it was first scanlated. I was probably 13 at the time. I had never read a serious manga which included death before... It made me cry for hours. I resolved to never read any BL ever again.
It's now 13 years later. I reread Birthday (and Ami Sugimoto's Dokusaisha Granada) every couple of years, and I feel like it means more to me every time. The character writing is raw and a bit lyrical, both boys feel real even though we only get to know them for a short time. The art style, which once seemed old-fashioned due to the strong sense of realism, actually feels more like modern BL, where styles are more diverse than they were in the 2000's. The story itself, which features a mostly unspoken and surprisingly mature relationship, is just perfect. Over the years since I read this manga, I feel like I have been both boys - I've been suicidal, I've loved a friend, I've lost loved ones, I've been in hospital. I've grown alongside it, and it has always been here for me when I've needed it. Birthday isn't just one of my favourite manga; it has changed me for the better.
My only sadness is that no one has licensed it. If you read this, please ask your favourite company to license it and Dokusaisha Granada. (Or any of Ami Sugimoto's work other than Animal X, tbh.) You'd have my everlasting gratitude.
If you like this manga and are looking for something with a similar vibe, try X-Day by Setona Mizushiro, 3-Ban Sen no Campanella by Kyoyama Atsuki, or Asterisk by Torino Shino.