Jewish holidays follow the Hebrew calendar, so their dates appear to move around each year. Now Google Calendar should be better able to help keep track of them. Google has rolled out the ability to select the Hebrew calendar in the web version of Google Calendar. This will enable users to see Hebrew dates alongside their usual ones.
The Hebrew calendar is a lunisolar calendar that bases its months on the lunar cycle and its years on the solar one. This means dates don't typically line up with the Gregorian calendar many of us are accustomed to.
To enable the calendar, just go over to settings and enable "Hebrew calendar" as an alternative calendar under the General tab.