Ah, distinctly I remember it was in the bleak December
This is the first line of the second stanza. By setting the poem in December, Poe takes advantage of the long periods of darkness at that time of the year, as well as the "dreary" weather. He aims to make the tone of the poem one of darkness and desolation; setting it in the middle of June would not have had quite the same effect!
The full text is available at the link I have posted below.