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.
The diopter (or power) of a lens is defined as 1000/f, where f is the focal length of the lens measured in mm. Thus, a lens of 50mm has a diopter of +20 = 1000/50, and a +4 diopter close-up lens has a focal length of 250mm = 1000/4. A close-up lens with larger diopter value has higher magnification. However, not all lens makers mark their close-up lenses with diopter values. For example, Canon has 250D and 500D, where 250 and 500 are focal lengths of the 250D and 500D, respectively.