"Hellatus" is the fannish term for the horrible periods of time during the year Supernatural is not airing. It became a popular fan term for the hiatus between seasons or breaks in the season, between Season 3 and Season 4, when Dean actually was in Hell.
The main Hellatus is over the summer usually from May - September. There is a shorter winter break from November/December to mid-January. There is often a break of one or two weeks around October, and again in March, mainly to ensure that certain episodes will air during the November and February Sweeps Weeks.
Due to the Writers Guild of America Strike, Season 3 broke from December 13th 2007 to January 31st 2008, and then again from February 21 to April 24 2008.
On February 5th 2010, it was announced that would be a 6 week hiatus after the airing of the February 11th episode until March 25th. The longer hiatus partly due to the Winter Olympics being held in the last 2 weeks in February.
Of course, our Hellatus is the cast and crew's only holiday, with filming for each season running from July to the end of March, so April - June is their only break for the year.
For dates of filming each season see Filming dates