Tisha B'av (the ninth day of the Jewish month of Av) is the national day of mourning for the Jewish people.
The final meal before Tisha B’Av (“Seudah Mafseket”) is eaten in a state of "mourning", sitting on the floor, eating a piece of bread and a hard boiled egg with some ashes.
Eating round foods such as eggs, beans or lentils as a sign of mourning is an ancient tradition. When Abraham died, his grandson Jacob was making red lentils for his father Isaac to eat. It was these lentils that Esau purchased from Jacob in exchange for his birthright. The reason such foods are eaten by mourners is to remind us that death and mourning is part the circle of life. Just as bad times come around in our lives, so too good times are sure to follow.
Also these foods are completely closed and have no openings. This reminds us that the mourner also has no "mouth" and is left to sit and mourn their loss without having to speak if they do not wish to.
The Seudah HaMafseket is not eaten as a communal meal. Everyone present sits apart on the floor, so not to cause them to say Birchat HaMazon (the blessing after the bread meal) as a group, which increases happiness, but rather alone.
When Erev Tisha B’Av is Shabbat, however, these practices do not apply since no mourning is allowed on Shabbat. A proper “Shabbat” meal should be eaten at Seudah Shlishit, and this meal may even include meat and wine.
May this be the last Tisha B'Av our nation must endure before the rebuilding of the third and final Temple in Jerusalem.