Autumnal Equinox Day, or Shubun no Hi (秋分の日), is a national holiday held on September 22 or 23, and rarely on September 24. It has been an official holiday since 1948.
The exact date on which this holiday falls depends on astronomical measurements, which is the reason why it varies from year to year. Because of that, the official date is not announced until February of the previous year.
The last time the Autumnal Equinox Day fell on the 24th was in 1979, and it has been calculated that the next time we will see it fall on the 24th will be in 2103 and 2107.
Date of the Autumnal Equinox for the coming years:
2018 → September 23
2019 → September 23