Designating food for Shabbos during weekday shopping:
Living with the verse “Remember Shabbos and sanctify it” if one sees a nice portion of food during the week he is to designate it for Shabbos. If he then finds a nicer portion, he is to eat the previous portion during the week and designate the nicer portion for Shabbos. This was the custom of Shamaiy, and so agreed Hillel that it should be the practical directive for others. However, Hillel himself, as a result of his great trust in G-d would wait [until Friday] to designate food for Shabbos, saying that certainly G-d will grant me the greatest portion [on Friday] in honor of Shabbos.
 Admur 242:10