Every single book in the series so far has been released in late summer or autumn.
The first four books were released in August, November, August and October of their respective years.
A Dance With Dragons was the earliest released in any year, coming out in July 2011. Expect The Winds of Winter to blow into bookshops around that time also.
Martin may have gone silent because he is indeed finally completing the manuscript.
source: express.co.uk