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.
If your lens is a 35-105 mm zoom, it's equivalent to having every possible focal length between 35 mm and 105 mm. Or if you just get two lenses, get the 28-70 mm and the 70-210 mm, and you then have all the focal lengths between 28 and 210 mm.