Natsu no, Ano Hi.

1. Natsu no, Ano Hi. (夏の、あの日。, That Summer Day); Comic Kairakuten 2015-09
2. Natsu no Ano Hi Gojitsu (夏のあの日後日, The Day After That Summer Day); Comic Kairakuten 2016-05

Note: Chapter 1 is compiled in Boku no Toshiue no Kanojo. Chapter 2 is compiled in Ochite Torokete.