The basics: A retrograde occurs when a planet appears to be moving backwards from our vantage point here on Earth. It isn't actually moving backwards, it just appears that way. Mercury turns retrograde most often because it's nearest to us, about 3-4 times per year, and lasts for several weeks. When a planet retrogrades, what that planet rules tends to be thrown off, along with what the sign it's retrograding in rules.
With Mercury retrograde, we experience a shadow and a storm. The shadow period starts when Mercury first hits the point in the zodiac it will turn direct at, and ends when it last hits the point in the zodiac it will turn retrograde at. Most people don't feel the effects of Mercury retrograde until it enters the storm period though. The storm period of Mercury retrograde is when Mercury is moving at its slowest, less than 40 minutes per day. Mercury is a fast-moving planet and hates it when he's moving slowly. There's usually two storm periods, one around the time Mercury turns retrograde, and one around when it turns direct.
The shadow of this Mercury retrograde begins October 18th and ends December 14th, lasting eight weeks. The first storm period will be from November 1st - 11th, and the second will be from November 24th - December 1st. The exact retrograde begins November 6th at 6:04PM ET and ends November 26th at 5:48PM ET.
This retrograde begins in Sagittarius, and it'll be the last one in fire signs for a few years. Mercury turns retrograde early in the sign at 4 degrees, so November-born Sagittarians will feel the beginning of this retrograde most. This won't be a good time for schools/learning, especially colleges and universities. Travel could be impacted, with more delays/cancellations than usual, or something strange happening that halts travel. There may be more fighting amongst people of different cultures, clashing of beliefs. We could see a scandal with a writer or in the publishing industry. Legal debates could be going on, reconsidering different laws and whether or not they should stand.
Mercury retrogrades backwards into Scorpio on November 14th. Scorpio rules other people's money (like taxes, debts, loans, inheritances, joint finances) and transformations (like death, sex, pregnancy). Now the thing with this part is that Saturn is already in Scorpio by this time as well, so the problems that come up may be even bigger, as a result of irresponsibility. It wouldn't surprise me a bit to see a country or two collapse economically (or come close to it). This retrograde ends at 18 degrees Scorpio, so those Scorpios born around November 8th - 12th may feel the turn direct most.
For the rest of you, check the house(s) Mercury will be retrograding in of your own natal chart, and any aspects Mercury will make during its retrograde to your natal planets, to fully understand how it will impact you personally.
If you have Mercury retrograde in your natal or progressed chart, you may not feel the worst of this. Those individuals generally don't experience the difficulties of the retrograde because it feels more natural to them. Of course, chances are everyone else around them doesn't have the same aspect, so they can still experience difficulties brought on by others.
Mercury Retrograde in Sagittarius: December 2017
January 2018 New & Full Moons
December 2017 New & Full Moons
Jupiter in Scorpio: October 2017 - November 2018
Saturn in Capricorn: December 2017 - March 2020, July - December 2020
Saturn in Sagittarius: December 2014 - June 2015, September 2015 - December 2017
Saturn in Sagittarius for Each Sign