When to go shopping for Shabbos:[1]

[In general,] it is better to go shopping for Shabbos on Friday, which is Erev Shabbos, than to go shopping on Thursday.[2] However, this only applies to readymade foods that do not require preparation to be fit to be eaten, such as beverages and various types of delicacies and the like [i.e. readymade dips, canned foods, foods from a catering, Challahs, etc etc].[3] However, those foods which still require further preparation [i.e. cooking, grinding] it is better to purchase the foods on Thursday [or another day of the week if necessary[4], as opposed to Erev Shabbos].[5] [Thus, it is better for one to purchase the meat, fish, and vegetables that he will use for cooking, on Thursday. Even by a ready to eat food, if one knows that the food item will certainly be available on Friday for him to purchase it, however, if it may no longer be available for purchase on Friday, and it is needed for Oneg Shabbos, then it is better to purchase it on Thursday than to nullify the Mitzvah.[6] Likewise, all the above only applies if one knows that he will have time to go shopping on Friday for the ready-made foods that he desires. If, however, one may not have time to shop also on Friday, or there will be very long lines, then he should purchase all of the foods, including the ready-made foods on Thursday.[7] Thus, if one can only go shopping once, either on Thursday or Friday, it is better to go shopping on Thursday, unless one knows for certain that he will have more than enough time to do all the Shabbos preparations on Friday including the shopping. See Q&A regarding Thursday night.]

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[1] Admur 250:7; See Tosefes Shabbos 250:1; M”B 250:2; Kaf Hachaim 250:2-3; Piskeiy Teshuvos 250:1

[2] Admur ibid; M”A 250:1; Arizal in Peri Eitz Chaim Shaar 18:1-2; Shaar Hakavanos Inyan Leil Vav p. 61 “One needs to beware to buy all of his Shabbos needs on Friday, and not Thursday.”; Birkeiy Yosef 250:2; Kaf Hachaim 250:2

The reason: In order so it is more recognizable that one is shopping for Shabbos. [M”B 250:2] Alternatively, in order to perform an action of preparation. [Arizal ibid]

[3] Admur ibid; Tosefes Shabbos 250:1 in explanation of M”A 250:1

The reason: As by such foods there is no reason to purchase them earlier, and hence it is better to be purchased on Friday than on Thursday in order to emphasize that they are being purchased Lekavod Shabbos.

[4] Kitzur Shelah; Piskeiy Teshuvos ibid

[5] Admur ibid; Tosefes Shabbos 250:1; M”A 575:4; Kaf Hachaim 250:2 that if it will take plenty of time to prepare, then it is to be purchased on Thursday; See also Admur 242:11

The reason: The purchase should be done on Thursday in order so one have plenty of time to prepare these foods, as well as to perform all of the other Shabbos needs. [Admur ibid; M”A ibid; Tosefes Shabbos ibid]

[6] Birkeiy Yosef ibid; Shaareiy Teshuvah 250:1

[7] See Siddur Shelah; Piskeiy Teshuvos ibid

[8] See Shevet Halevi 9:129; Rav SZ”A, brought in Yom Tov Kehilchaso 25 footnote 12 p. 496; Rebbe in Sichas 29^th Elul 5747