The six weekdays and Shabbat are interconnected. Just as one has a body and a soul, so, too, the week has a body (the work days) and a soul (Shabbat). Just as a wholesome person’s body and soul work together harmoniously – the body receives spiritual inspiration from the soul and in turn serves as the vehicle for the soul’s expression – so, too, an ideal week is one in which Shabbat and the weekdays are closely linked. During the week we prepare for Shabbat, thus elevating and sanctifying the weekdays, which draw their value from Shabbat.
The Sages recount that Shammai would always eat in honor of Shabbat. When he saw a fine animal he would say: “This is for Shabbat.” If he found an even better one, he would eat the first one and leave the better one for Shabbat (Beitza 16a). In other words, all week he would eat lower quality food, always saving the best for Shabbat. In this way, whenever he ate he honored Shabbat and considered how he could honor and sanctify it. Hillel possessed a different trait. All his actions were undertaken for the greater glory of God. He would make a point of honoring and sanctifying the weekdays as well as Shabbat, thus fulfilling the mandate to “Bless God every day” (Tehilim 68:20). Therefore, when good food came his way during the week he would eat it, trusting that just as God made sure he had good food during the week, He would make sure that he would have even better food for the holy Shabbat. Thus Hillel honored and valued each day (Rashi ad loc.; Ramban, Shemot 20:8).
According to halakhic authorities, even Hillel agreed that for most people it is preferable to act like Shammai and designate the best food for Shabbat. However, Hillel’s trust in God was exceptional, so he was certain that God would provide him with better food for Shabbat. One who does not possess this degree of faith must follow Shammai and honor Shabbat by designating the best food for Shabbat (MB 250:2).
Nowadays, this law is almost irrelevant. There is such a vast selection of food in contemporary supermarkets at all times that there is no reason to think that if one eats a particular food during the week, he will not be able to find food as good for Shabbat. Therefore, today one must simply plan his shopping such that the foods for Shabbat will be the best.
While shopping for Shabbat, it is most proper to articulate that “this shopping is to honor the holy Shabbat,” thus fulfilling the commandment of Zakhor (Ramban op. cit.). It is also good to taste the food being prepared for Shabbat in order to season it correctly and enhance the pleasure of Shabbat (MA 250:1 in the name of Arizal; MB 250:2).