A speaker enjoys a lovely day by getting thoroughly drunk, and then, in that loosened state, starts wondering about his own head. The poem is only six short lines, but it manages a full little scene: good weather, a drink, and a woozy bit of philosophizing.
The repetition of "very pleasant" is the funniest part. The first time it describes the day; the second time it seems to describe the drinking, or the drunk himself. The speaker is so content that he repeats himself, which is exactly what a happy drunk might do. The mood is relaxed, a bit silly, and warmly self-mocking.
The ending gives it some depth. The "holes" and "merry spigot" in his head suggest eyes, ears, mouth, and a tap that pours out drink or talk. He puzzles over what they mean, as if his own body were a strange machine. It's a playful image, but it also hints at a real question about what we are and how we take in the world.