Newfag Summer is a period from May 2007 to the present day when 4chan's internet traffic suddenly doubled, implying there was a mass migration of newfags into the imageboards come summer vacation in most high schools. This period was characterized by extremely high turnover rate for new topics (with maddeningly record breaking rates in /b/, forcing moot to make a topic later in the year announcing new regulations to deal with the "declining quality of /b/") though the troubles plagued the other large boards as well, especially /v/ and /a/. Newfag summer was also a time of massive meme hemorrhaging, where, much to Anonymous' horror, very much hated members of notable anime communities (like AnimeSuki or GaiaOnline) started displaying a familiar knowledge of memes and started to use them in common speech in an attempt to look "cool" and "hip". Underaged b& rates also skyroketed, as did trollings from oldfags and newfags (who idolized the oldfags) alike. Most 4channers acknowledge newfag summer to be one of the most trying periods in the board's history, and parallels were drawn with the "unending summer" of Usenet and the "endless June" of 1983 in Higurashi no Naku Koro ni.
After moot enacted some changes following 4chan's four year birthday, such as stricter throttling of posting speed on /b/, the impact of the [Fag|newfags]] has lessened somewhat, but their presence is nevertheless felt. Another factor that has reduced the impact is that some newfags learn to fit in and get on the path to oldfagdom.
School letting out and many regulars going on vacation is bound to have at least some impact each year. It is not known if 2008 will have such a big impact as the summer of 2007, as much of the potential fail may have been unleashed in '07.
UPDATE: It did have an impact, and has reduced the overall strength of /b/ and anonymous a five-hundredfold.
Another factor to Newfag Summer were the recent news coverage on FOX and G4.