A big date is the designation for a display which, compared to the conventional date display, is enlarged substantially.
Although the visible element of a "big date" watch movement is a larger, more-readable date window, it is the technical means that defines this complication. All "big date" movements use separate elements to show the "tens" and "ones" portion of the date. This is often implemented with discs, (overlapping, concentric, or adjacent), which move at different rates through a complex set of gears.
A side-effect of many big date mechanisms is relocation of the date window from the edge of the dial (often among the markers) to elsewhere on the dial. Many big date watches emphasize this feature with a large window at 12:00. Such is the case for the prototype example from A. Lange & Söhne, which was first presented with the Lange 1.