Although there is a certain amount of preparation for Shabbat that should go on all week, the primary time to prepare is on Friday, as is stated: “But on the sixth day, they prepare what they have brought in” (Shemot 16:5). Just as in the desert first thing Friday morning the Jews gathered the manna that had fallen the previous night, so too it is a mitzva to be industrious and to prepare for Shabbat on Friday morning (SA 250:1). It is an admirable custom for women to wake up early on Friday, bake challah bread for Shabbat, and perform the mitzva of hafrashat ĥalla (Rema 242:1).
Even though there is a mitzva to shop early Friday, one should not do so before Shaĥarit. Similarly, if one normally studies Torah after praying, he should keep this routine and then immediately purchase food and prepare for Shabbat. One may go shopping before Shaĥarit only if there is a real concern that if he waits there will not be food for Shabbat left in the store (MB 250:1).
Aĥaronim have written that it is preferable to buy food for Shabbat on Friday than Thursday, since it is more apparent on Friday that the purchases are being made to honor Shabbat. There is also another reason: in the past, when there was no refrigeration, there was no way to keep food fresh. Therefore, to make sure that Shabbat food would be of superior quality, one had to shop and cook on Friday. However, if there are foods that might not be available on Friday or whose preparations are very time-consuming, they should be purchased on Thursday; this was true then and is certainly true now (MB 250:2).
If one is faced with two possibilities – cooking all the food on Friday and being exhausted Friday night, or cooking on Thursday, refrigerating the food, and entering Shabbat calmly – he should finish cooking on Thursday and leave some small things to be done on Friday. This is because the primary mitzva is to honor and enjoy Shabbat, so it is important that he enter it in an alert and relaxed state of mind.
There are families where there is so much tension involved in trying to get everything done before Shabbat that Friday becomes an aggravating day, full of arguments and anxiety. As it were, there are satanic forces at work, trying to foment anger and disagreement and prevent Israel from welcoming Shabbat properly and peacefully. The Talmud tells of a couple who would have horrible fights each week before Shabbat. R. Meir was visiting their neighborhood, and for three weeks he visited them on Friday, until he made peace between them. He then heard the voice of a prosecuting angel lamenting: “Woe is me, for R. Meir has chased me out of this home” (Gittin 52a). In order to avoid giving this angel an opening, one must organize Shabbat preparations so well that he welcomes Shabbat joyfully and peacefully (this is also implied by Ezra’s takana [ordinance]; see below, section 4).
There is a holy custom to finish all Shabbat preparations by midday on Friday and then to rest and study Torah in the afternoon. All who follow this custom are privileged to welcome Shabbat joyfully and peacefully and become truly aware of the neshama yeteira given them on Shabbat.