The vast majority of the action in the sagas occur during the spring and autumn, when the sun is doing a fairly even split between light and dark. When there are references to actions during the summer or winter, they're usually described as being done in daylight (mainly during summer, though rarely during the winter, as in Njála) or at night (almost exclusively during the winter, as there's no real 'night' in the summer)

As far as mentioning it is concerned, there wouldn't really be any need for the difference to be brought up, as the audience would be familiar with the conditions during each season.

Obviously, the sunlight hours are only abnormal to non-Icelanders. You can find plenty of Icelandic phenomena which are never mentioned in the Sagas, such as glaciers, northern lights and volcanic eruptions.

Generally, the Sagas are very short on descriptions. Clothes, weather, seasons, time of day, looks, etc are usually only mentioned if necessary for the plot. Darkness is occasionally mentioned, or the time of day.