Doki Doki Shattered Time
Most mods install on top of a fresh copy of DDLC, free from ddlc.moe. Follow the author's instructions, and keep a clean copy of the game.
About this mod
After being convinced by Amy, MC performs a ritual to travel back in time to discover the secret of how Natsuki died and figure out how to save her. Due to the ritual, MC is doomed to repeat the same cycle until he saves Natsuki's life. Essentially he is trapped in a time loop until he came save her life. All the while, he begins to learn things about Natsuki and himself.