For thousands of years, the Jews celebrated their New Year around the beginning of spring, just like everybody else in West Asia. Passover was probably port of this old New Year celebration. Sometime before 100 AD, though, the Jews split off from this ancient tradition and began to celebrate a different new year, the holiday of Rosh Hashanah, at the beginning of fall, instead of at the beginning of spring. People celebrated by eating apples and honey, sweet foods to bring a sweet year. They kept on celebrating Passover, but they began to think of Rosh Hashanah as the New Year.
Nobody knows why this change happened, or even exactly when – after the Babylonian Captivity? During the Maccabees’ revolt? Maybe during the First Jewish Revolt? Probably the Jews had always had several New Year holidays, just like we have the beginning of the new school year in September, even though the calendar New Year is in January. This was just a change of emphasis.
Yom Kippur came at the end of the Rosh Hashanah new year festival, and it was a ritual to purify the Temple in Jerusalem to get it all clean and ready for the next year. The priests prayed and put on fresh clothes and killed a bull. They burned incense and killed two goats, as well as doing other things.
People began to use Yom Kippur as a chance to purify themselves for the new year as well – just as Americans often make New Year’s resolutions, Jewish people tried to start fresh by forgiving their enemies, returning things they had borrowed, and working out any feuds between families. This helped keep the peace by preventing long feuds from getting started.
Another way of getting a fresh, clean start for the new year was to fast – to not eat anything – for the whole day of Yom Kippur, and many Jews did fast for Yom Kippur.