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 August 28th and ends October 24th. The exact retrograde begins September 17th at 2:09PM ET and ends October 9th at 10:57AM ET. The first storm period is from September 10th - 24th, and the second storm period is from October 6th - 14th.
This Mercury retrograde occurs entirely in air sign Libra, who’s getting kind of beat up by the Universe right now as we come out of Venus retrograde, Libra’s ruler, September 6th and then go right into Mercury retrograde in your sign. Sorry Libra! (I say that as a Libra too!) Libra rules relationships, and Venus rules relationships, so we’re in the middle of some major relationship energy since July and through mid-October. Everyone has to evaluate their relationships and what’s working, what isn’t, who stays, who goes. We see the continuation of breakups, critical relationship energy, and people having a hard time getting along. We can see lots of fights and misunderstandings and people just don’t want to be tactful, diplomatic, or treating people well.
Libra rules justice, so we can see issues related to justice come to the forefront (as if they haven’t been already). It can be more obvious, more important to us, and we want to work on having more equality, fairness, and respect. Though we may not act very civil at times!
We can be more uncompromising with Mercury retrograde in Libra, lack harmony and peace around us, and can see people in general as well as governing bodies being less-than-nice to each other. Though it is a good time to pick peace talks back up that have fallen by the wayside.
Libra is vacillating, and so we can be more indecisive; Libra is superficial, and so we can care less about what’s underneath; Libra is easily unbalanced, and so we can find ourselves feeling completely out of whack.
There are a few interesting things going on with this retrograde. The way it starts and the way it ends is probably going to be pretty wild. The beginning of this retrograde will be square (hard aspect) Pluto in Capricorn, which can incite some major rage, some major struggles over power and control, and we can get along the least at the onset of this retrograde.
Then to end the retrograde, Mercury will be at 0 degrees Libra, called the Zero Degrees Aries Point, which is a point of critical energy. Astrologer Celeste Teal calls it a "spectacular out rush of energies... and involvement with the world at large". With this in Libra, it’s about how we are with one another, how we interact and relate and connect, and we have even more focus on our relationships to end this retrograde.
This Mercury retrograde makes 2 major aspects while in retrograde, a square to Pluto on September 24th (which we feel in orb at the start of the retrograde through the 25th), and a sextile (easy aspect) to Saturn on October 6th.
The sextile to Saturn can help us get a little discipline and focus during the wonky retrograde, a brief time to get some clarity before the retrograde ends a few days later.
An extra note: Venus retrograde ends on September 6th at 14 degrees Leo, and Mercury retrograde begins September 17th at 15 degrees Libra, which means if you have any planets or points between 13 and 16 degrees, it will be impacted by BOTH the end of the Venus retrograde AND beginning of the Mercury retrograde, and there will be a lot of energy and focus on that planet/point during the first half of September.
The turn retrograde begins at 15 degrees Libra, so those of you Libras born October 8th - 10th may feel the turn retrograde most. The turn direct occurs at 0 degrees Libra, so those of you Libras born before September 25th may feel the turn direct most.
Another extra note: If you have any planets or house cusps at 0 degrees Aries, Cancer, Libra, or Capricorn, the end of this retrograde will trigger the Aries Point energy in your chart, so pay special attention to it!
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.
Suggested Reading: The Mercury, Venus, and Mars Retrogrades page for general interpretations