Isshuukan Friends. (One-Week Friends.)
The story revolves around Yūki, a boy who wants to become friends with his classmate and desk neighbor Kaori.
However, she gently declines his friendship, saying, “But, my... Memories of my friends disappear in a week...”
Even so, Yūki wants to become her friend, and so the two become friends over and over again as the weeks pass.
[Source: Anime News Network]