The principal announced a surprise assembly.

This joke plays on a delightful twist of expectations and the universal student dream of a day off. The initial setup uses "surprise" in the typical sense of an unexpected event, like a sudden school assembly. However, the punchline cleverly reinterprets "surprise" from the student's perspective. Their surprise isn't about the assembly itself, but rather their astonishment that it *wasn't* a holiday, which they clearly had their heart set on.

The humor here comes from this misdirection (Review) and the relatable experience of being a student. Who hasn't, at some point, wished for an unexpected day off from school? Assemblies, while sometimes fun or informative, often interrupt the school day, and for many students, the ultimate interruption would be no school at all. This joke taps into that common longing, highlighting the often-divergent priorities of school administration and the student body with a simple, chuckle-worthy contrast.