What is the earliest Gregorian date on which Yom Kippur can fall?

The earliest Gregorian date on which Yom Kippur can fall is September 14.

Yom Kippur is fixed in the Hebrew calendar as the 10th day of Tishrei, rather than by a fixed Gregorian date. Because the Hebrew calendar is lunisolar while the Gregorian calendar is solar, the holiday moves through a limited range of dates each year.

September 14 is the earliest possible Gregorian date. The date has occurred in modern times, including in 1899 and 2013. At the other end of the range, Yom Kippur can occur in October, which is why relying on a single familiar date can be misleading.

A common mix-up is to confuse Yom Kippur with Rosh Hashanah. Rosh Hashanah begins on Tishrei 1, while Yom Kippur comes nine days later, on Tishrei 10. The interval between the holidays is part of the Ten Days of Repentance, also called the Ten Days of Awe.