Siman 300

It is stated in the Gemara (119b): "A person should always set his table on motzaei Shabbat, even if he only needs a kezayit. And hot water on motzaei Shabbat is a malugma (remedy), hot bread on motzaei Shabbat is a malugma. For Rabbi Abbahu, they would prepare a third-born calf on the conclusion of Shabbat, and he would eat a kidney from it. When his son Avimi grew up, he said to him: Why should you waste so much? Let us leave over a kidney from the eve of Shabbat. He left it over, and a lion came and ate it," thus far the words of the Gemara.

The reason for this meal was written by the Rambam in chapter 30: in order to honor the Shabbat at its entrance and at its departure, meaning that this is like escorting the king with the honor of a meal, and therefore this meal is called "Melaveh Malkah." Therefore, it is customary to say liturgical poems and songs to escort the Shabbat, in the manner that one escorts the king at his entrance and at his departure (Beit Yosef), and therefore it is proper to say the songs after Havdalah (Taz). And they wrote that the sages said that there is one organ in man and its name is "Niskoy," and some call it "Luz," and it does not benefit from any eating except from the eating of Motzaei Shabbat in order to escort the Shabbat (Levush), and from it man was formed. And when he dies—this bone does not decay, and even if they place it in fire it will not be burned, and in a mill it is not ground and with a hammer it is not shattered, and from it man will live at the time of the resurrection, and its root and essence are from the essence of the heavens (Eliyah Rabbah in the name of Matteh Moshe).

Many are not careful regarding this meal, but one who fulfills it, his reward is great, and all who fear G-d exert themselves to fulfill it and eat a kezayit. It is proper to eat meat for one who is able to eat it, and if not—he should eat fish, as this is also an important food, or good fruits if he has them. He should eat them with bread, and one who cannot eat bread at all—he should fulfill it with mezonot and fruits. If he continued the third meal into the night and ate a kezayit at night—he can also fulfill the Melaveh Malkah meal through it (Eliyah Rabbah). And in Tosafot Shabbat (118a, s.v. Ve-ha) it is explained that even for a poor person traveling from place to place, they must give him a meal for the conclusion of Shabbat; see there.

The Maharil had a tallit for Shabbat, and he would fold it every Motzaei Shabbat in order to involve himself in a mitzvah immediately (Magen Avraham), and so is the custom to fold the tallitot on Motzaei Shabbat. And they were accustomed to drink hot water on Motzaei Shabbat as it is a remedy, as written in section 1, and they should be careful not to make a sign for Motzaei Shabbat for one who requests a debt or a gemilat chesed, and says 'it is Motzaei Shabbat' and it is not a good sign, and he transgresses 'you shall not practice divination' (Sanhedrin 66a), and it is of the ways of the Amorites as it is found in the Tosefta of Shabbat chapter 8, see there, and therefore it is forbidden to say so, and 'you shall be wholehearted' etc.