Lunar months, solar years
Each Hebrew month begins with the new moon and has 29 or 30 days. Twelve months fall about 11 days short of a solar year, so a leap month — an extra month of Adar, called Adar I — is added in 7 years of every 19 (years 3, 6, 8, 11, 14, 17 and 19 of the cycle). That keeps Passover in the spring, as the Torah requires.
The calendar has been calculated by fixed rules, rather than by watching the moon, since about the 4th century. Years are counted from the traditional date of creation, so 2026 is mostly the year 5786 or 5787 AM.
Six possible year lengths
Ordinary years have 353, 354 or 355 days and leap years 383, 384 or 385. Between 1950 and 2100 the counts are: 353 days — 14 years; 354 days — 36 years; 355 days — 44 years; 383 days — 24 years; 384 days — 8 years; 385 days — 23 years.
Why Rosh Hashanah never falls on certain days
The new year, Rosh Hashanah (1 Tishri), is sometimes postponed by a day or two so that later holidays do not land on awkward days — Yom Kippur, for example, never falls next to the Sabbath. As a result Rosh Hashanah is never on a Sunday, Wednesday or Friday: in every year from 1950 to 2100 it falls on a Monday, Tuesday, Thursday or Saturday. In the Gregorian calendar it falls between 5 September and 5 October.
The next Rosh Hashanahs: 12 September 2026, 2 October 2027, 21 September 2028 and 10 September 2029.
Days begin at sunset
A Hebrew day runs from sunset to sunset, so every holiday begins on the evening before the date shown on a Gregorian calendar. The Sabbath runs from Friday evening to Saturday evening.