From Echochi Scans:
Two families are tied together by a curse. Every 100 years, one person from each family must share the same fate for exactly one year. The pair will be subjected to 12 misfortunes where one will get hurt while the other must sustain their injuries in return.
When Kagura Ikumi was 7, his parents died in an accident. Since then, he has been living an ordinary life. Until the day of his 17th birthday where he meets a mysterious transfer student, Ichijyou Chihaya. However, this fateful encounter between them is only the start of their intertwined fate. For the next year, the two of them will be forced to face 12 misfortunes on the 13th of every month, a curse that has never left any previous pair alive.