In just four short lines, a speaker imagines being invited to dinner by a fly. The joke is in the reversal: instead of a person inviting a person, an insect has set the table, and the speaker, apparently alone, accepts the company without complaint. The title, "Tablemates," makes the fly an equal, someone to share a meal and a drink with.
The speaker seems relaxed, even a little wistful. "This time of day" suggests a regular hour, perhaps evening, when the day's work is done and there's room for small pleasures. Singing "anything frivolous" to a glass of gin hints at a loose, unguarded mood, where nothing needs to be serious or impressive. Singing to the drink, rather than to a person, also suggests solitude, though the poem treats it lightly rather than sadly.
The overall mood is quiet, playful, and gently comic, with a touch of loneliness underneath. It's a tiny moment of contentment found in an ordinary, slightly absurd scene.