The Shulhan Aruch (Orah Haim 249:2; listen to audio recording for precise citation) writes that on Erev Shabbat one may not eat a larger meal than what he is accustomed to eating during the week. Due to the importance of eating festive meals on Shabbat, the Sages wanted to ensure that a person begins Shabbat with a hearty appetite, and to that end they forbade conducting large meals on Friday. Regular-sized meals, the Shulhan Aruch adds, are permissible on Erev Shabbat, but only until the ninth hour of the day. Thereafter, one should refrain from eating even regular-sized meals until the onset of Shabbat. The Kaf Ha'haim (Halachic work by Rabbi Yaakov Haim Sofer, 1870-1939) writes that one should preferably refrain from eating sizeable meals already from Hasot midday on Friday.
This Halacha applies even to meals involving a Misva, such as a Berit Mila, Pidyon Ha'ben or engagement party. Such meals are permissible on Erev Shabbat, but when a meal of this nature is held on Friday one should ensure to schedule it for the morning hours, and not during the afternoon. Similarly, fund-raising events involving a meal should not be held on Friday afternoon; even if the purpose is to raise funds for a worthy cause, the function should take place only in the morning hours on Erev Shabbat. Beyond the issue of beginning Shabbat with an appetite, there is also the concern that participation in these events on Friday afternoon will interfere with one's Shabbat preparations. Hence, even worthwhile social functions held on Friday should be scheduled for the morning hours, rather than for the afternoon.
As a general rule, everybody knows his particular appetite and should arrange his eating on Friday accordingly, such that he will be able to eat heartily when Shabbat begins.
The Shulhan Aruch records a practice of the "Anshe Ma'aseh" particularly pious Jews to fast every week on Erev Shabbat, in order to ensure to begin Shabbat with an appetite. The work "Da'at Hochma" mentions another reason for this practice, namely, that these individuals sought to spend the day of Erev Shabbat engrossed in introspection and soul-searching. The letters of the word "Shabbat" also form the root of the word "Teshuva" ("T.Sh.V."), indicating that Shabbat is to be observed as a day of repentance. Accordingly, the righteous members of the nation would fast on Friday in order to prepare themselves for proper Teshuva on Shabbat.
Although few people observe this practice today, it nevertheless underscores the importance of beginning Shabbat with a hearty appetite.
Likewise, the Gemara in Masechet Gittin (38b) tells of two families in Jerusalem that were, tragically, eliminated on account of sins they habitually committed. One family regularly held large, festive meals on Erev Shabbat, and the other would eat a meal on Shabbat during the time when the Rabbi delivered his weekly lecture to the community. This unfortunate story further reinforces the importance of ensuring a hearty appetite on Friday night.
Summary: Halacha forbids eating on Friday a larger meal that one normally eats during the week. After the ninth hour, it is forbidden to eat any large meal. Preferably, however, one should not eat large meals after midday on Friday. Hence, social functions involving large meals, including events such as a Berit Mila, Pidyon Ha'ben or fund-raising function, should not be held on Friday afternoon. In general, one should schedule his eating on Erev Shabbat in such a way that he will have a hearty appetite when Shabbat begins.
