What will you do when the last sky comes?
Tsui no Sora is a 1999 KeroQ game written by SCA-Ji. It is sometimes called one of three big denpa games (along Sayonara wo Oshiete and Jisatsu no tame no 101 no Houhou), and it would be remade/reimagined in 2010 as Subarashiki Hibi.
(The following review may contain spoilers for Subahibi; I would recommend to first play Subahibi it you are interested in it.)
(You can also check the summary of differences between Tsui no Sora and Subahibi.)
Minakami Yukito is our generic protagonist going to school with his osananajimi Wakatsuki Kotomi. However, upon arriving at school on Monday, July 13th, they discover that the previous day their classmate Takashima Sakuro had committed suicide. Almost immediately after that, in the class a rumor starts circulating predicting the end of the world on July 20th, and implying that Zakuro’s suicide might have had something to do with that. And soon after that, until then unremarkable and bullied Mamiya