The fifth season of Game of Thrones will debut on HBO in the United States on Sunday, 12 April. In the UK, the show will likely air on Sky Atlantic the following evening.
In the meantime, the fourth season will be released on DVD and Blu-Ray on 16 February. IMAX fans in the USA will also get to see an exclusive trailer for Season 5 of the show starting on 23 January, when HBO airs the final two episodes of Season 4 in IMAX cinemas. I suspect the trailer will very quickly appear online shortly thereafter.