Well, to a large extend it is related to number crunching. When the date advances, the game engine must evaluate all event triggers for all eligable objects that can receive events.
Those are a lot of combinations.
Now, depending on the Windows installation, DirextX the video driver and a few other things, the mouse arrow is either driven (low level) directly by the video hardware, or it is triggered through the application that has the foreground focus.
It sounds that on your system the latter is the case. When the day advances, the game engine evaluates all those event and AI related things. During that time, front end user interface interaction is frozen (you cannot, and should not, want to interact with a game universe that is in transition from one turn to the next, as it would lead to inconsistent results). If the movement of the mouse arrow on your system is setup to be handled through the main application (aka the game), then the game won't come around to it until the calculations that are needed for the transition to the next game day are completed. During that time, the mouse arrow will seem frozen, and unresponsive to movements of the mouse itself.