This is a warning not to brag about what you expect to happen tomorrow. This can be stated clearly. AT: "Do not speak proudly about your plans for tomorrow" (See: [[rc://en/ta/man/translate/figs-explicit]])
Something happening on a certain day is spoken of as if the day were to bring that event. AT: "what will happen on a day" or "what will happen tomorrow" (See: [[rc://en/ta/man/translate/figs-metaphor]])