I think with the Time Machine release, it was announced in July that it would come out in late September, and got pushed back to Novmber. Clockwork Angels came out on the correct date that was originally announced. I believe it was announced in September that it would come out November 19th, which it did.
This is a bigger release though, and it's also 99.8% guaranteed to be their last live release of something recent(maybe one day we'll get Hemispheres and Permanent Waves era stuff). Add the fan documentary aspect to it, and I'm sure they will want to take their time making it the best they possibly can.
Take into account the manufacturing of the packaging, the distribution and marketing, the advertising, etc...and I don't think it will be released by Christmas. Usually if something get's released in December, it's within the first 2 weeks.
Also...2016 makes sense, just for the sake of having a Rush release in the bands 42nd year after the first release. A limited number of Steelbook editions like Beyond The Lighted Stage would be cool too.
Just my 2 cents
Edited by BloodofTheZodiac, 13 September 2015 - 02:12 PM.