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 September 14th and ends November 10th. The exact retrograde begins October 4th at 1:02PM ET and ends October 25th at 3:17PM ET. The first storm period is September 28th - October 10th, and the second storm period is October 22nd - 30th.
This retrograde begins in water sign Scorpio. This will be the last retrograde in a water sign for a few years, so cheer up water signs! In Scorpio, we can see problems with transformations and other people’s money. Transformations can mean things like sex or death, so these things could have a problem at the start of this retrograde. We can see the return of former lovers, seem morbid or focused on the dark side of life, and either have a hard time being serious, or are way too serious and need to get bopped upside the head. We’re overly emotional (since water signs rule our emotions), and we’re moody to the extreme, going from one extreme emotion to the next like it’s nothing and normal. Not nothing, not normal!
Mercury will retrograde backward into air sign Libra on October 10th at 1:26PM ET, so he won’t be retrograde in Scorpio for very long. The time around the 10th is the most important since Mercury will be at the beginning of one sign and the end of another, so the energy is amplified, and we can really see people fighting (or even war kind of fighting) around this time.
Libra rules committed relationships, so we can simultaneously see relationships falling apart like that (*snap of the fingers*), and people reunite that we didn’t think would (or should). For those who break up, communication is likely the culprit, and since it’s during a retrograde, the break up may not be permanent, so don’t do anything drastic (like drive their car off a cliff). For those who get back together, you remember all of the things you loved about each other, but since it’s during a retrograde, the reunion may not be permanent, so don’t do anything drastic (like buy a wedding dress).
Libra also rules fairness, equality, justice, compromise, diplomacy - all of that goes out the window. We’re fighting like cats and dogs, and don’t want to give an inch, let alone a mile. We can say the wrong things at the wrong times to the wrong people, and are less than tactful. This could especially be the case when Mercury conjuncts (is in the same spot as) transit (moving) Venus on October 17th, and we can stick out foots in our mouths like crazy, or try to kiss and make up, even just temporarily.
The one good thing about this retrograde is Mercury doesn’t make any difficult aspects during its course, so phew for that - but not so fast. The end of this retrograde will be opposite the lunar eclipse occurring October 8th in Aries, so we could see some wackiness from that toward the end of the retrograde. And the start of this retrograde will be conjunct the solar eclipse occurring October 23rd, but that could make things a little easier at the start. Getting out guards down so it can rip our hearts out!
The turn retrograde begins at 2 degrees Scorpio, so those of you Scorpions born October 27th and earlier may feel the turn retrograde most. The turn direct occurs at 16 degrees Libra, so those of you Librans born October 8th - 11th 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.