Background Seasonality in youth suicide has been speculated to be associated with the school calendar, as it tends to increase at the beginning of the academic year or after a long break, but robust empirical evidence remains scarce.

Methods We examined the nationwide death records in the Vital Statistics of Japan to investigate the seasonal patterns of suicide among youth. Our data set included 108 968 suicides by individuals who died at 6–26 years of age between 1974 and 2014 in Japan. The daily frequencies of death were plotted against the Japanese school calendar, which has little regional and temporal variations. We also estimated a Poisson regression model to uncover the cyclical patterns of suicide deaths.

Results We found that the frequencies of suicide by middle school students (ages 12–15 years) and high school students (ages 15–18 years) sharply increased around the dates when a school session began in April and September. These tended to be low during school breaks. The results of regression analysis suggested middle school students were more than twice as likely to die by suicide when the summer break ended and the second semester began, compared with the baseline week in July. Similarly, the frequency of suicide for high school students also increased by ∼40% at the end of the summer break. Importantly, no such pattern was found for those aged 18–26 years.

Conclusions Our findings strongly indicate that the cyclical pattern of youth suicide is closely related to the school calendar.