Speak to the Children of Israel and tell them these are the Festivals that they shall keep holy (23:2)
– [For] six days, work may be performed, but on the seventh day, it is a complete rest day (Shabbat Shabbaton), a holy occasion; you shall not perform any work. It is a Sabbath to the Lord in all your dwelling places. (23:3)
Rashi wonders why Shabbos is inserted into the middle of the parsha of the Festivals (moadim).
The Vilna Gaon comes up with a fascinating explanation that explains the pasuk in a different vein. On all the Festivals certain types of melachos are permitted (‘ochel nefesh‘), whereas on Shabbos all melachos are forbidden. However on one yom tov no melacha is permitted – Yom Kippur – which is also known as Shabbat Shabbaton – the same terminology that the Torah uses for a regular Shabbos. Thus the Vilna Gaon explains the pasuk like this;
– Speak to the Children of Israel and tell them these are the Festivals that they shall keep holy (23:2)
– Six days “of these” a melacha is permitted ( “these” are first and last days of Pesach(2), one day Shavuos (3), one day Rosh Hashana (4), one day Succos (5), one day Shmini Atzeres (6) [these are the days that are Yom Tov ‘mideoraisa‘ which are still observed today in Israel]) however the seventh is the holy of holiest – no melacha is permitted (Yom Kippur (7) [not even ochel nefesh])!