What term describes a particular day specified as the time something will happen?

The term that best describes a particular day specified as the time something will happen is "date." A date is a specific point in time often denoted by day, month, and year on a calendar. It provides an exact reference for when an event or occurrence is scheduled to take place, such as a birthday, meeting, or deadline.

While the other options relate to time management, they do not specifically denote the exact day. A schedule outlines a plan for when tasks will occur, a calendar is a system for organizing days, and a deadline refers to the latest time by which something must be completed but does not specifically indicate a singular date. Thus, "date" is the most accurate choice.