According to The Telegraph:
The precise date [of the 2016 advert] is not yet known, but the retailer has said it will be early November. Earmarking Friday, November 4 might be a good idea. Note it down! In PEN!
In the past few years the retailer has chosen to release the advert via YouTube at 8am on the first Friday of November, and last year  the first televisual airing was during Gogglebox on Channel 4 that evening.
To add to the suspense, last year  store bosses released a 10-second teaser alongside a mystery hashtag which appeared during a Sunday night episode of The X Factor the week before - so keep your eyes peeled and don't go and get a cup of tea in the ad breaks.