"Where is it winter in June?"
Translation:Júniusban hol van tél?
There is no such rule about beginning a sentence. It has to do more with how definite that noun is. Here, "június" is not the subject (which is another way of saying there is an added suffix), and it is not one specific June. And the names of months may be special in that the article is often optional. So it may be the combination of those.
For example, if we replaced "június" with another noun, say, "ország" (country), you would have to use the article. But with "június" it is incorrect indeed.