All to do with a Jewish holiday, I understand. Surely they have enough
non-Jewish employees to cater for the rest of us?

It has nothing to do with the amount of non-Jewish employees. If the owner
is Jewish,
he is not supposed to conduct business (interpret this as make money) on
some of the
more important holidays (Rosh Hashanah, Yom Kippur, First and last days of
Succoth, First days of Passover) and Sabbath (Saturday) regardless of if he
has non-Jewish
help. They are still making money for him.