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 1st and ends November 27th. The exact retrograde begins October 21st at 6:29AM ET and ends November 10th at 4:12PM ET. The first storm period is October 15th - 26th, and the second storm period is November 7th - 15th.
Scorpio rules transformations. It's change on a grand scale, and it's not easy. We could see lots of transformations taking place around the world with cultures or countries. Scorpio rules power and control, and those who feel they've been oppressed, that they're being controlled by a group or a government that is too powerful, may rise up against them. Scorpio can be destructive, so it probably wouldn't be a quiet rebellion. You'd have to take notice of it. It would be a good idea to watch your temper, and not let other people set you off. Sensitivities may run high, and we could overreact.
Scorpio rules the dark side, and we might be a bit darker than usual, or be completely turned off by anything serious. We'll likely sway from one extreme to the other, since Scorpio rules the extremes. Issues related to intimacy could come up, and we as societies and individuals may want to connect, but have problems doing so. Issues related to sex could also come up, with news related to STD's or pregnancy coming up.
Scorpio also rules other people's money, so we could see issues with taxes (like there aren't enough!), debts, loans, inheritances, joint finances, business partnerships. We have to do a better job of not only managing our own finances, but also dealing with the finances that involve other people. Tax scandals? Bank scandals? Major business scandals? Countries defaulting? Bad financial news?
Secrets are also Scorpio's domain, and we could learn about something that someone (or many someones, or a government) has been hiding from us. It comes out into the light, and we have to find a way to deal with it. It may be unsettling, but it's better to know. We just probably won't think it's better to know when it comes out.
This retrograde will actually conjunct (be in the same spot as) transit (moving) Saturn in Scorpio. We had Mercury retrograde backward into Scorpio last year, but it was in late Scorpio while Saturn was in early Scorpio. This time, they'll be at the same point in Scorpio on October 29th, so this might be the most important date of the retrograde when we could see the biggest event(s) occur. All we can do is prepare for the worst and hope for the best.
The turn retrograde begins at 18 degrees Scorpio, so those of you Scorpios born November 8th - 12th may feel the turn retrograde most. The turn direct occurs at 2 degrees Scorpio, so those of you Scorpios born October 24th - 28th 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.