There are five holidays that were established in the time of Moshe, commanded by God, with a few other important days connected to them:
Rosh Hashanah, Yom Kippur, Sukkot (plus Shmini Atzeret), Pesach (with the last day of Pesach), and Shavuot.
All of those, including those days connected to other holidays, are observed by one extra day far from Jerusalem. For instance, outside of Israel, Passover is 8 days comprised of two chag-days at the start (e.g. two days with a seder) and two more at the end of that week considered like a holiday, but in the Land of Israel it is 7 days comprised of only one chag-day at the start (so only one seder) and one more holiday at the end of that week. There are holidays established later, like Hanukkah and Purim both after the First Temple period, and days of mourning that are one day only, but the only single-day Torah-mandated holiday is Yom Kippur.
In ancient times, the Jewish calendar was primarily based on the sighting of the new moon to determine the beginning of each month. Since the moon’s appearance could not be precisely predicted in advance, messengers were sent from Jerusalem to Jewish communities outside of Israel to inform them when the new moon had been sighted. However, due to the time it took for these messengers to travel, there was uncertainty about the exact start date of the new month for communities far away from Jerusalem. comprised of two chag-days at the start and two at the end of that week.
To ensure that they did not miss the important holidays, communities would keep an extra day. That explains why the rest are observed two days, but not Yom Kippur’s exceptional status. Perhaps not too surprisingly, it is because it is a fast day, and it was deemed too strenuous for any community to be expected to go without food and water for roughly 49 hours. This also touches on an important principle of Yom Kippur: the most important element is the fasting element commanded in the Torah, the only non-rabbinic fast.
Yom Kippur is the most important day of the holiday, this notion is deeply rooted in Jewish tradition and religious teachings. Yom Kippur, also known as the Day of Atonement, is a time of deep introspection, repentance, and seeking forgiveness for sins committed throughout the year. Fasting on this day is seen as a way to cleanse the soul, remove distraction and material joy, and demonstrate sincere remorse for one’s wrongdoings. Yom Kippur offers someone a chance for a clean slate, so the opportunity to fast, give gratitude, and repent should be met with joy.
While hearing the Torah reading and praying in a minyan (a prayer quorum) are essential elements of many Jewish holidays, Yom Kippur stands apart in its emphasis on the fast. The fast is considered a personal and spiritual endeavor, allowing individuals to focus solely on their relationship with God and their quest for forgiveness. It is not undermining their importance that the fast takes priority, but if for some reason an individual must choose between fasting and going to a synagogue, the fast should be considered first. Always talk to your rabbinic authority first who can assess the situation more specifically.
Yom Kippur is a time to focus on self-actualization, betterment, and communal involvement. Because their was a concern having two days would detract from this on an extreme level, it is universally one day. This means it is all the more imperative to make this day count.