How the Hebrew calendar works
Months begin with the new moon and have 29 or 30 days. A common year has 12 months and 353, 354 or 355 days; a leap year adds a month, Adar I, before Adar, which is then called Adar II, and has 383, 384 or 385 days. Years 3, 6, 8, 11, 14, 17 and 19 of each 19-year cycle are leap years. The year number changes at Rosh Hashanah on 1 Tishrei in the autumn, while the months are counted from Nisan in the spring. Rules that keep Rosh Hashanah off certain weekdays make the lengths of Heshvan and Kislev change from year to year.
Holidays begin at sundown
A day in the Hebrew calendar runs from sunset to sunset. Every holiday in the list therefore begins on the evening before the first date shown and ends at nightfall on its last day, and the tool says on which evening it starts. Sunset time depends on where you are, so for the exact time of candle lighting or the end of a fast, use a local calendar or ask your community. The minor fasts, which begin at dawn, are not listed here.
Israel and Diaspora dates
Some festivals are kept for a different number of days in Israel and outside it, in the Diaspora. Passover lasts seven days in Israel and eight outside it, Shavuot is one day in Israel and two outside, and Simchat Torah falls on the same day as Shemini Atzeret in Israel and on the following day elsewhere. Where the count differs, the tool shows two labelled rows and does not prefer either. Communities also differ in their customs, so the list is a guide to the calendar, not to practice.
Which holidays are listed
Rosh Hashanah (1 and 2 Tishrei), Yom Kippur (10 Tishrei), Sukkot (from 15 Tishrei), Shemini Atzeret (22 Tishrei), Simchat Torah, Hanukkah (eight days from 25 Kislev), Purim (14 Adar, or 14 Adar II in a leap year), Passover (from 15 Nisan), Shavuot (6 Sivan) and Tisha B'Av (9 Av, moved to Sunday 10 Av when 9 Av falls on Shabbat). In Jerusalem, Purim is kept one day later as Shushan Purim.
How the dates were checked
The conversion follows the published rules of the fixed Hebrew calendar. It was compared with the browser's built-in Hebrew calendar for every day from 1900 to 2200, and its holidays with the US Naval Observatory for 2025 to 2028 and 2046, and with Hebcal.com for 2026 and 2027 in Israel and the Diaspora. That testing found that the Hebrew calendar built into some browsers is one day off from November 2045 to November 2046, so this tool uses its own calculation, which matches the Naval Observatory there. This is general help, not religious guidance; for questions of practice, ask your own rabbi or community.