A speaker sits on a grave and eats a meal, and that small, slightly shocking act carries the whole poem. The title points to the Day of the Dead, a time for remembering those who have died, so the meal can feel like a quiet way of keeping the dead company. Eating on a grave blurs the line between the living and the buried, and the speaker seems comfortable there rather than frightened or disrespectful.
The second stanza turns to the sky. "The day mixed with evening" suggests dusk, when light and dark run together, much as life and death do in the scene below. "Routed and red" is the most striking phrase. "Routed" suggests the day is being defeated and driven off, and the red could be a sunset or something bloodier. The mood is calm but somber, with a hint of menace. Judging by the imagery, the speaker accepts that endings, whether of days or lives, arrive in color and in company.