A speaker sits down to an absurdly huge meal: a whole horse, squeezed into a single sandwich. It's funny and a little grotesque, and the title, "Feast day," makes it sound like a celebration, though the greed on show feels closer to hunger that can't be satisfied.
The second stanza turns from the sandwich to something larger. A man, it says, can "pepper every love" and "nutmeg whole rooms / full of despair." Spices are things we add to make food bearable or delicious, so the poem seems to suggest that people flavor their sadness and their affections in the same way, with small touches that change the taste of everything. The word "whole" matters here: despair fills entire rooms, yet a little nutmeg is supposed to handle it.
The mood is odd and playful on the surface, with a darker undertone. The speaker seems to be coping through appetite and humor, making something enormous manageable by turning it into food.