"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. Hellatus is often marked by outbreaks of fandom drama!
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.
In 2014, a three week break between episodes 17 and 18 was dubbed #ShowerHellatus by fans on twitter because of the images of Dean in the shower in the promo for the episode 9.18 Meta Fiction.
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 May - June is their only break for the year. The writers and executive producers get little break between seasons. Season 14 was shortened to 20 episodes specifically to allow Jared and Jensen a longer break between seasons.
The break between Season 14 and Season will last from April 25th to early October 2019 and will be the longest Hellatus on record.
For dates of filming each season see Filming dates