A unit test exercises an API and verifies that it returns correct values for correct inputs, and handles incorrect inputs correctly. In your case, I'm guessing that your API will produce an HTML table given (at least) a date.

A simple unit test might feed that API several dates, comparing the resulting HTML to known-good reference copies.

To make this work will probably require that you get your data access abstraction right, so that you can pass a stubbed database in for testing purposes. Or, you can structure the calendar object such that it's populated by a separate agent. Use a dummy agent for populating with test data, and a real agent (which you write separate unit tests for) for accessing the database.