The day after Christmas was also the traditional day on which the aristocracy distributed presents (boxes) to servants and employees  a sort of institutionalized Christmas-bonus party. The servants returned home, opened their boxes and had a second Christmas on what became known as Boxing Day.
300 mm. For a 35 mm camera with a diagonal of 43 mm, the most commonly used normal lens is 50 mm, but focal lengths between about 40 and 58 mm are also considered normal. The 50 mm focal length was chosen by Oskar Barnack, the creator of the Leica camera.
Focal Length. The technical definition of a standard lens is one whose focal length roughly matches the diagonal or the film or image sensor. For a standard full-frame 35mm camera this gives a focal length of around 43mm. In reality, the actual focal lengths chosen by manufacturers tend to be slightly longer than this. For a 35mm sensor, 50mm is the most common standard lens, although some companies do sell lenses which are closer to 43mm focal length.