Rashi on Leviticus 23:3/רש"י על ויקרא כ״ג:ג׳

1ששת ימים SIX DAYS [MAY WORK BE DONE BUT THE SEVENTH DAY IS THE SABBATH OF STRICT REST] — What relation is there between the Sabbath (the day that is invariably fixed by God as the holy day) and the festive seasons (the times of which are variable, being dependent upon the proclamation of the Sanhedrin)? But by putting both into juxtaposition Scripture intends to teach you that he who desecrates the festivals is regarded as though he had desecrated the Sabbath, and that he who keeps the festivals is regarded as though he had kept the Sabbath (Sifra, Emor, Section 9 7).