Why is Scandinavia known as 'the land of the midnight sun'?

Because Scandinavia is so far north that in summer the days are very long and the nights are very short. Whilst it may not be possible to actually see the sun at midnight it will still be quite light in the very late evening. Conversely, in winter, the nights are very long and the days very short. It is then the "Land of the Midday Gloom"