Here is his blog post.
And here are some choice excerpts:
THE WINDS OF WINTER is not finished.
Believe me, it gave me no pleasure to type those words. You’re disappointed, and you’re not alone. My editors and publishers are disappointed, HBO is disappointed, my agents and foreign publishers and translators are disappointed… but no one could possibly be more disappointed than me. For months now I have wanted nothing so much as to be able to say, “I have completed and delivered THE WINDS OF WINTER” on or before the last day of 2015.
But the book’s not done.
Nor is it likely to be finished tomorrow, or next week. Yes, there’s a lot written. Hundreds of pages. Dozens of chapters. (Those ‘no pages done’ reports were insane, the usual garbage internet journalism that I have learned to despise). But there’s also a lot still left to write. I am months away still… and that’s if the writing goes well. (Sometimes it does. Sometimes it doesn’t.) Chapters still to write, of course… but also rewriting. I always do a lot of rewriting, sometimes just polishing, sometimes pretty major restructures.
And then later…
Unfortunately, the writing did not go as fast or as well as I would have liked. You can blame my travels or my blog posts or the distractions of other projects and the Cocteau and whatever, but maybe all that had an impact… you can blame my age, and maybe that had an impact too…but if truth be told, sometimes the writing goes well and sometimes it doesn’t, and that was true for me even when I was in my 20s. And as spring turned to summer, I was having more bad days than good ones. Around about August, I had to face facts: I was not going to be done by Halloween. I cannot tell you how deeply that realization depressed me.
It’s clear that 2015 didn’t go as GRRM had planned, and the summary is this. He’s always been a slow writer, more specifically one that writes in streaks. When he gets cold, he doesn’t make much progress. He has said in an interview, he wishes he continued with The Winds of Winter when he was finishing ADWD because he hasn’t been able to get on a hot streak like that one since. And the situation is complicated by all of the distractions in his life. Those distractions are a choice of his own. So there we have it. GRRM said his publishers could probably have the book out within 3 months of him finishing, but unless he has it finished by September, that means we won’t have The Winds of Winter in 2016.