day is the the side of an planet or other object that is in the light of an star. For example: if earth turns the continent of Europe in the light of our sun, Europe is in day. Not on every planet is a day as long as on earth(23h 56m 4.09053083288s). On mars, one day is 24 hours, 39 minutes, and 35.244 seconds. On mars a 'day' is called: a sol, and more than one sol : sols.