Let’s say rosh Hashanah. It varies it’s date every year, but if the Jewish calendar is a fixed amount of days, wouldn’t it be earlier and earlier or later and later every year?
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The dates don’t change. For example, Rosh Hashanah always begins on the first of Tishrei. But this is on the Jewish calendar which isn’t aligned with the Gregorian calendar, which is why it is on different dates with respect to the Gregorian calendar.
The Hebrew calendar uses lunar months, which are based on the phases of the moon, e.g. the new moon is always the first day of the month, and months are always 29 or 30 days long. The Gregorian calendar’s months are fixed to the solar year, so the two never line up 1:1.
Since there aren’t a whole-number of lunar months in a solar year (or even a whole number of days in one lunar month) the Hebrew calendar adds a thirteenth leap month in seven out of every 19 years to keep it roughly in sync with the solar calendar. That’s why Rosh Hashana (or the first day of Tishrai) always falls between September 5 and October 5.
The Hebrew calendar is based on the lunar cycle instead of the solar cycle. So instead of the normal year having 365 days the Hebrew calendar have about 354 years with leap months added to make up the difference. This means that each month starts with a new moon. You can see remnence of this in the Cristian holidays related to Easter. In the Hebrew calendar the Cristian Easter selebration takes place on the same day every year.
Changing dates is btw not exclusive to the Jewish calendar. The Muslim Ramadan changes its date every year, too, as well as certain Christian holidays, like Easter and Pentecost.
Jewish calendar uses lunar cycles to define months—12 of them that are 29-30 days long. It then adds in a leap month in certain years to re-center the calendar to the solar cycle.
Do they change their date according to the Jewish calendar? That’s the real question.
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