Though the now recognized british poster found huge popularity during the Christmas of 2005, it was originally produced in 1939 for the Ministry of Information and was only to be distributed should the Nazis succeed in an invasion of Great Britain. The third in a series of three it was designed as a motivational poster for the British Government.
Printed and stored, the poster didn’t see the full light of day until the year 2000, when a copy was discovered by a bookseller Stuart Manley and gained popularity shortly after it was featured as a newspaper supplement Christmas gift idea.
Below is the original image along with a parody found on ffffound. While the typography is clearly off the mark, the British stiff upper lip sentiment surely gets turned on its head.

RELATED LINKS & RESOURCES
http://news.bbc.co.uk/2/hi/uk_news/magazine/7869458.stm
http://ww2poster.wordpress.com/2009/04/05/1939-3-posters/
http://en.wikipedia.org/wiki/Keep_Calm_and_Carry_On