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Why did October 4, 1582, become October 15 in countries adopting the Gregorian calendar?

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Kalendarium (“Calendar”) by Regiomontanus — calendar First complete printed title page for the *Kalendarium* (“Calendar”) by Regiomontanus, 1476.

The transition from October 4 to October 15, 1582, was a necessary adjustment to correct a cumulative error in the Julian calendar.

The Julian calendar overestimated the length of the solar year by 11 minutes and 14 seconds. Over centuries, this discrepancy caused the calendar to drift out of sync with the seasons, specifically shifting the vernal equinox away from its traditional date of March 21. Because the date of Easter is calculated based on the equinox, this drift became a significant concern for the church.123

To restore the equinox to March 21, Pope Gregory XIII issued the papal bull Inter gravissimas, which mandated that 10 days be omitted from the calendar. October was chosen for this adjustment to avoid interfering with major Christian festivals. Consequently, in countries that adopted the new Gregorian calendar, the day following October 4, 1582, was designated as October 15.23