Lit. Meal
Seudah Shlishit is the third meal on Shabbat. A seudat mitzvah is a meal associated with a mitzvah, like a wedding feast or a party at a brit milah. On Purim, a special seudah is held in the afternoon.
This immersion is for prayer leaders who have some experience writing prayers and want to receive feedback and hone their skills as liturgists in a supportive environment.