On hebrewbooks.org I found a sefer call Oros Shabbos by R' Shlomo Wahrman that I am enjoying immensely. I remember years ago R' Wahrman's seforim (he has a number of volumes titled She'eris Yosef) were very popular among the YU / Morasha Kollel chevra, no doubt in part because in addition to being a gaon atzum, he is a Rosh Yeshiva at HANC (Hebrew Academy of Nassau County) -- there was a certain feeling that he is one of us, or at least knows where we are coming from. Anyway, I thought it would be nice to do a series on some of the torah in his sefer so that I remember it better (the self-serving motive here) and so that you can share in the enjoyment. (I'm open to comments if you think this is not a good idea or a waste of time, and it goes without saying that summaries are only a taste and not a replacement for the sefer and any mistakes in the posts are mine alone.)
The Shulchan Aruch (O.C. 250) writes that there is a mitzvah on Friday to prepare for Shabbos. R' Wahrman discusses three different possible sources for this din:
1. The meforshim on the S.A. (see GR"A) point to the gemara in the beginning of the second perek of Kiddushin (41), where, in the context of discussing "mitzvah bo yoseir m'bshlucho"(Rashi: because one gets greater schar for personally attending to the mitzvah), the gemara mentions how various Amoraim would personally attend to Shabbos preparations in their home. Given this context, it seems that the mitzvah of preparing for Shabbos is no different than any other mitzvah which should preferably be done personally rather than through an agent.
2. Rav Wahrman points out that there may be an additional element to the mitzvah of preparing for Shabbos that is unique to hilchos Shabbos. Rashi elsewhere explains (Shabbos 119a) that the reason for the elaborate preparations for Shabbos is because it shows the importance of the day, similar to the preparation one would make for one's rebbe or an important guest. Preparing for Shabbos is not just a means to an end, a way to make sure one can fulfill the mitzvah of oneg Shabbos properly with cholent and kugel, but rather it is an end in its own right -- the very act of preparation demonstrates the importance of Shabbos. Personal involvement is necessary not just as a function of mitzvah bo yoseir mbshlucho, a means of getting extra schar, but rather personal involvement is an essential component of the etzem hamitzvah because it is this personal attention which demonstrates kavod for Shabbos.
3. Aside from the divrei kabbalah elements of kavod and oneg Shabbos there is a d'oraysa kiyum in preparing for Shabbos based on the pasuk of "V'haya bayom hashishi v'heichinu es asher yavi'u" (see Biur Halacha).
One nafka minah between these sources might be when preparation should be done. The M.B. (250:5) writes that it is better (if possible) to make preparations on Friday for Shabbos rather than on the preceding day. If the mitzvah of preparing for Shabbos was just a means to the end of ensuring one has food, then it would seem to make little difference if cooking is done on Thursday, Friday, or any other day. However, if preparing for Shabbos is itself a fulfillment of kavod Shabbos, then it is understandable that preparations should be made davka on Friday when they stand out as being undertaken specifically for the honor of Shabbos.
This factor of kavod can also help resolve another difficulty. The Chavos Ya'ir writes that even though kavod habriyos overrides mitzvos derabbanan, the Amoraim put aside their kavod to personally be involved in making ready for Shabbos because of its great kedusha. If preparing for Shabbos was just a means to an end, then it would not seem to matter how it got done, whether personally or through an agent, and there would seem to be little reason to sacrifice one's kavod to do the mitzvah. However,if personally being involved is what defines preparations as being l'kavod Shabbos, if personal involvement is part of the definition of the etzem hamitzvah, obviously it is impossible to delegate the chore.