The argument

to
CalendarChange converts between Western (Gregorian) dates and Jewish dates. The Gregorian date of Israel's independence is May 14, 1948:
Because the Jewish calendar has a leap month preceding Passover,
Mathematica uses a numbering starting with Nissan, the month of Passover:
Israel's independence was declared on the 5th of Iyar, 5708.
In
Mathematica numbering, Tishrei, the first month of the Jewish year, is number 7.
By definition, Yom Kippur cannot fall on a Friday or Sunday:
Because of the leap month, Jewish dates can vary by up to a month in the Western calendar. The Western and Jewish anniversaries of Israel's independence differ greatly:
The Jewish calendar follows a 19-year cycle. Western and Jewish anniversaries will correspond almost exactly every 19 years: