Read quickly, the poem sounds like a toast that has slipped out of ordinary language. The speaker says "I sat za zen," then "we sat zen za," and the words rearrange themselves as if the group is settling into a rhythm, or as if the drink is loosening their tongues. "Zen" suggests quiet meditation, but here it sits beside "many healths" drunk to a "Mr. Zan," so the calm seems to have turned into a cheerful, slightly blurry gathering.
The last two lines change the tone. "There was nothing at all left / of this day's work" can be read in two ways: the day's labour has been wiped from the mind, or the day has simply vanished into the evening. Judging by the wording, the speaker feels relaxed and content, perhaps a little giddy. The mood is playful and warm, with a hint of the emptiness that follows when effort is let go.