Translation:December is the last month of the year.
Usually yes. "des" is the genitive version of the masculine definite article and genitive indicates possession (as well as being used after certain prepositions).
der.. Monat = nominative masculine definite article= the month
des Jahres = genitive masculine definite article = of the year
The word der/die/das usually means the. In the genitive case the masculine word for the "der" is changed to "des". In the genitive case it still means the but also indicates posession.
e.g. "der Mantel des Mannes" = the man's coat
der Mantel = the coat
des Mannes = of the man i.e. the coat belongs to the man. Genitive is used to indicate that the man owns the coat.
Still not accepting it 10/2016. I will report it too. I mean, it's not like it's wrong in any way that I can think of.
Here is a table of them.
It should help if you are looking to know which version of definite or indefinite article to use or how to decline your adjectives with strong weak or mixed inflection. In order to make good use of it you will need to know the gender of your nouns and which case each part of the sentence is in. I would also suggest making sure you are solid on your prepositions as they often direct you into accusative, dative or genative when you might not otherwise use that case.