I see it started in 1997 and its up to 7 volumes but still ongoing. Does anyone know why its like that?

well....i just searched and it seems like its still ongoing but there are 8 volumes currently in japan
and there is a sequel as well
here(i hope you can understand french ).....no pages are available in english...weird.

Toume typically works on more than one project at the same time, and as a result Yesterday isn't regularly serialized. It runs in Business Jump, which is a monthly publication, but Toume doesn't publish a chapter every month. She seems to finish enough to top off a volume every year and a half or so, but at the same time works on things like Hatsukanezumi no Jikan or Genei Hakurankai.

I haven't been in Japan recently, so I'm not certain if the 8th was the last volume and it's just finishing with the sequel, but I suspect it's getting close to that.

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That's not true, Sing Yesterday for me is only up to volume 7 (EX doesn't count), volume 8 isn't even released yet in Japan. Delcourt is publishing the manga and in french and they are at the same level as the japanese edition, so volume 8 would have to be released in Japan first before being published in France. Hope the volume 8 is the last one, cause I have been holding off reading this for years waiting for this manga to finally be done.