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.
It is a common mistake to describe normal lenses by comparing focal distance to sensor or film diagonal - a normal lens shows the proportions like the human eye. Tele lenses compress them, and wide angle lenses expand them, as you take the proportions of far and near objects. 50mm is about the average for the human eye and it changes nothing if your camera crops or not.