Se'udah Hamafseket

From RRW

 The final meals before the fasts of Tisha B'AV and Yom Kippurt

1. The most obvious parallel is that both fasts last for a full day - about 25 hours - as opposed to other fasts which last from early morning until night.
2. Both Final Meals traditionally follow the Minchah Service.
3. Both Final Meals have several customs and rituals unique to these meals.

What makes these meals so special?

Nearly every holiday has a special mealtime ritual. The most obvious is the Passover Seder. On the New Year, we eat special symbolic foods to start the year off right. At Sukkos we eat in the Sukkah. On Shavuos, we have the custom of eating dairy meals.

We cannot possibly engineer a meal symbolizing either Yom Kippur or Tisha b'Av. The problem is obvious. They are both FULL DAY fast days. 

That is where the final meal comes in. The reason we eat it after Minchah is to connect as closely as possible to the upcoming fast day. This explains, perhaps, why we specifically recite the confession at Minchah before  the meal on Yom Kippur eve.

This final meal is somewhat festive since Yom Kippur is, after all, a Yom Tov. We eat Challah, some have honey and we eat meat - although it is wise to eat bland and easily digested foods to prepare for the fast.

On the other hand, Tisha b’Av is a day of mourning. Mourners coming back from the burial normally DO have a special Mourner's Meal. Since we cannot eat on Tisha b’Av, this Mourners Meal takes place before the Fast.

Therefore, these final festive meals function as substitutes for the meals that should have been consumed on the days themselves, but could not be eaten due to the fasts.

The Talmud teaches us that whoever eats on Yom Kippur Eve for the sake of the Yom Kippur Fast Day is considered to have fasted Both Days.