"Tag", can mean both "friday" as well as "the 14th of december of 2018". And the other way round: if you want to translate the german sentence: "Welcher Tag ist heute". That could be translated as: "What day do we have today". The answer could be: "It is friday the fourteenth of december," or shorter: "it is the fourteenth", or: "Friday". Context is important. You probably know the person asking the question, you already know what is important to them.