Because that's not grammatically correct in written English. You can get away with this verbally (with a distinct pause after month), but you're doing the equivalent of a colon (The month: June.) or answering your own question (The month? June.). Given that the DL example is using the 'de' preposition in Spanish, the 'of' preposition is clearly what it's looking for as a translation.
In speaking English we just say June or July and not the month of June or month of July it is understood to be a month we are talking about. Now if your girlfriend's name is June and you say I am meeting June later....well we know you are talking about your novia, comprende? Adios amigos Frank