(a) I do believe that in your link (an Australian newspaper) the headline lacks the "a," and a gentleman is quoted in the article using an "a." After I post this, I will check again. If I am mistaken, then I'll edit this post.
Actually it's used three times, (1) in the headline, (2) at the beginning of the article, and (3) in a quote.
So, not only is the indefinite article omitted in the heading, but also in the text.1) Coldest start to winter on record in Adelaide
2) ANYONE shivering in Adelaide this morning had good reason to do so - it was the coldest start to June on record.
3) "I think it's the coldest start to a June that we've ever had at Kent Town," Mr Rowlands said.
(b) Regarding "The theater [Americans only spell it as "theatre" when they
want to be very fancy or elegant!!!] opened in May," and "We got married in December," why, yes, I do believe there are cases where [when?] It's funny that you should mention that. I was wondering about the same thing myself. an
"a" is possible:
Tony: Are you married?
Joe: Not anymore.
Tony: Oh, I'm sorry.
Joe: Yeah, it was one of those cases. We got married in a December
and divorced in the following January.
So, is the December in your example different from the previous one because you added the 'January' follow-up?