In just three lines, this haiku sets up a strange and unsettling scene. It's a sad, rainy night, and a surgeon hands "us" a cake that has already been cut up. A cake should mean celebration, but here it arrives from the hands of a surgeon, and the cutting suddenly sounds like something done to a body. The title, "Vivisection," which means cutting into a living creature, makes that link hard to ignore.
The speaker never says how they feel, but the mood is heavy. The rain and the word "sad" set the tone, and the dark joke of the cake makes it sharper. It seems to suggest a family or group waiting for news, or coping with an operation, and being handed something meant to comfort them that only reminds them of the knife. Judging by the imagery, the feeling is a numb, uneasy mix of grief and absurdity, where ordinary politeness sits awkwardly beside something frightening.