Dining-In & Mess Night
A formal, members-only dinner with a junior officer assigned to needle the guests, a bowl of deliberately terrible punch waiting for anyone who breaks the rules, and centuries of British regimental-mess custom underneath it.
How it actually works
A Dining-In is a formal dinner for the members of a unit or mess — service dress or mess dress, a receiving line, toasts, and a president of the mess running the evening under a set of unwritten "mess rules" (no talking business, no leaving the table without permission, and so on). A junior officer or senior enlisted member is designated "Mr./Madam Vice" and is expected to be quick-witted, since part of the job is answering the president's ceremonial jabs and proposing toasts. Violate a mess rule and you can be sentenced to drink from the grog bowl — a punch mixed from whatever the mess feels like combining that night, sometimes with things floating in it. The custom of "grog" itself comes from the Royal Navy, where a sailor's rum ration was diluted with water specifically to cut down on drunkenness; the modern grog bowl inverts that intent entirely. The lineage runs back through British Army and Royal Navy regimental mess customs, adopted early by American officers' messes and formalized differently service by service.
Two things get conflated. First, "Dining-In" and "Dining-Out" are not the same event — a Dining-In is members only, while a Dining-Out includes spouses and guests, and the rules (and the grog bowl) get toned down accordingly. Second, there is no single DoD-wide official script for either. Individual academies and commands publish their own guides — the Naval Academy and the Navy's Mess Night Manual, for instance — but the specific customs, toasts, and grog-bowl theatrics vary unit to unit. If your mess night looked different from a buddy's at another command, you were both doing it right.
Traced through British Army and Royal Navy regimental mess customs; adopted by American officers' messes from the Revolutionary War era onward, formalized independently by service.