You may notice that the number of days of some holidays do not accord with what the Bible specifies. In most cases, we celebrate one more day than the Bible requires. There is an interesting reason for this additional day.
The Jewish calendar is lunar, with each month beginning on the new moon. The new months used to be determined by observation. When the new moon was observed, the Sanhedrin declared the beginning of a new month and sent out messengers to tell people when the month began. People in distant communities could not always be notified of the new moon (and therefore, of the first day of the month), so they did not know the correct day to celebrate. They knew that the old month would be either 29 or 30 days, so if they didn’t get notice of the new moon, they celebrated holidays on both possible days.
This practice of celebrating an extra day was maintained as a custom even after we adopted a precise mathematical calendar, because it was the custom of our ancestors. This extra day is not celebrated by Israelis, regardless of whether they are in Israel at the time of the holiday, because it is not the custom of their ancestors, but it is celebrated by everybody else, even if they are visiting Israel at the time of the holiday.
Rosh Hashanah is celebrated as two days everywhere (in Israel and outside Israel), because it occurs on the first day of a month. Messengers were not dispatched on the holiday, so even people in Israel did not know whether a new moon had been observed, and everybody celebrated two days. The practice was also maintained as a custom after the mathematical calendar was adopted.
Yom Kippur is celebrated only one day everywhere, because extending the holiday’s severe restrictions for a second day would cause an undue hardship.
List of All Holiday Dates
Below is a list of all major holiday dates for the next five years. All holidays begin at sundown on the day before the date specified here.