On Friday one is meant to be preparing for Shabbat. The Sages forbade sitting down to an unusually large meal on Friday, because doing so would spoil one’s appetite for Friday night dinner (SA 249:2). It also would be disrespectful toward Shabbat, since it would make it seem like Shabbat meals and weekday meals are no different from one another (Pri Megadim ad loc.). Furthermore, it is possible that dealing with a large meal would get in the way of necessary Shabbat preparations (MA ad loc., quoting Rabbeinu Ĥananel). Even a se’udat mitzva (a ceremonial festive meal associated with a mitzva) such as a siyum masekhet (a celebration upon completing a unit of Torah study) may not be planned for Friday. However, before midday one may have a regular meal to celebrate a siyum.
The Talmud tells of a family that lived in Jerusalem and hosted elaborate meals on Fridays. Because of this sin, they first became impoverished and then died out (Gittin 38b).
When there is a se’udat mitzva that must take place at a specific time, such as a brit on the eighth day or a pidyon ha-ben (redemption of the firstborn) on the thirtieth, it may be held on Friday. Since the Torah designates a time for them, it is a mitzva to serve a big meal, and this does not detract from the honor of Shabbat. Nevertheless, it is proper to hold these celebrations before midday, both so as not to disrupt Shabbat preparations and so as not to spoil people’s appetites for the Friday night meal (SA 249:3; MB ad loc. 13 and 695:10; BHL s.v. “mutar”).
It is specifically eating a large meal that the Sages prohibit, but technically one may eat a regular meal anytime on Friday. Nevertheless, the Sages recommend not eating a meal with bread in the three hours before Shabbat, so that at the onset of Shabbat one will be hungry. One may partake of a snack of cake or fruit before Shabbat starts, as long as it does not diminish his appetite.
There were pious individuals who set very high standards for themselves and did not eat at all on Friday; they felt that if they ate they would reduce their appetite for the Shabbat meal. However, one who finds fasting difficult should not do so, as we are not supposed to enter Shabbat while suffering (SA 249:2-3; MB ad loc. 18). In any case, each person should plan his meals on Friday such that he will begin Shabbat hungry and will enjoy the Friday night meal.