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, 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.
This Mercury retrograde begins on December 3rd at 2:34AM ET and ends on December 22nd at 8:51PM ET. The shadow period is November 15th 2017 - January 10th 2018. The first storm period is November 29th - December 7th, and second storm period is December 19th - 28th.
Mercury madness! We're coming up on the last Mercury retrograde for 2017, and this one occurs entirely in fire sign Sagittarius. Sagittarius rules optimism, expansion, exploration, new experiences, and Mercury retrograde can mess with all of that. We can be unrealistically optimistic, or overly pessimistic and miss the optimism; we can expand too much in some ways, and not enough in others; we can explore places we shouldn't, and avoid the places we should go to; we can dive into experiences we should run from, and avoid experiences we should have.
Travel is often disrupted in some way with Mercury retrograde in Sagittarius (particularly flight), and we may see issues with colleges/universities (higher learning is Sagittarius-ruled), or legal issues come up (Sagittarius rules the law). We can be extra boastful, make promises we don't keep, break promises, and talk a big game that we can't back up. We can buy into false prophets, and we can be too defensive of our beliefs and not open-minded enough. Holy war, oh holy war . . .
This retrograde is problematic in a variety of ways, to say the least, but it's mostly focused right at the start. First, it begins at 29 degrees, which is the anaretic degree. The anaretic degree is the very last degree of any sign and a point of crisis energy, and Mercury will be anaretic in November 30th (before the retrograde starts) to December 5th (already retrograde). This can be difficult energy, and we may see a significant legal issue, accident energy may be high, and we may fight even more over beliefs (hopefully we can avoid some sort of attack, but I'm not holding my breath).
Second, the retrograde begins December 3rd, and we'll have a full moon that very same day, and the full moon occurs in Gemini, one of the signs Mercury rules (the Moon will be in Gemini to December 4th 3:37PM ET). Full moons are highly charged emotional energy, and with this being in one of the signs Mercury rules, that just becomes even more complicated. With Gemini and Sagittarius and Mercury all being communication rules, I'm thinking some sort of bad news, bad decision-making, bad ideas, bad plans. Maybe a combo!
Third, the retrograde begins conjunct (aligned with) Saturn, who is at the end of his tour of Sagittarius. So Saturn is wrapping things up, and Mercury retrograde comes along wanting to muck it all up - Saturn won't like that very much! This is exact December 6th (7:05AM ET), and there will likely be some paying of the piper, some last-ditch lesson, some complex karma coming upon us with Saturn. Maybe something with the law or with politics. (BTW, Saturn exits Sagittarius and moves into Capricorn December 19th - read the article on Saturn in Capricorn HERE).
So we've got a lot to manage right off of the bat, and we're going to have to work extra hard to get through it as best as we can! Extra patience, extra time - extra liquor?
Mercury retrograde will be at a critical degree (17 degrees Sagittarius) December 15th - 16th, and this may bring some difficult energy, though not as trying as the start, and we may just be extra annoyed. The Moon will be in the other sign Mercury rules, Virgo, December 8th (6:09PM ET) to December 11th (12:01AM ET), and this can be another difficult period of the retrograde (but again, likely not as trying as the very start).
This Mercury retrograde will make 4 major aspects during the retrograde, including the conjunction to Saturn. It'll also sextile (positive aspect) Mars December 6th (10:58AM), same day as the Saturn aspect going exact, so hopefully we can get some positive energy to help us wade through the difficulties and push forward with energy and enthusiasm, the Mars way.
Mercury will conjunct the Sun on December 12th (8:49PM ET), called the inferior conjunction, and this can usually be a positive aspect providing an opportunity to start something (occurs at 21 degrees 14 minutes in Sagittarius, so see if that's doing anything to your chart to see if you can use the energy for yourself).
Mercury conjuncts Venus on December 15th (9:09AM ET), and we can strive to get along better, connect in different ways, and find some middle ground (or indulge away our troubles).
The retrograde ends square Neptune, though this doesn't go exact. We may come out of the retrograde in a bit of a fog, unsure of how to proceed or feeling that we're not seeing everything clearly. We need to let it lift before we make any big decisions.
Extra: Mercury retrograde also squares (hard aspect) Chiron on December 10th, and Chiron is dubbed the wounded healer who represents our greatest wounds and how we can heal and grow. Chiron ends its own retrograde December 5th square Mercury retrograde, so we may be focused on what we need to heal from, re-open an old wound, feel extra sensitive, and need to be careful with our words and rhetoric.
The retrograde begins at 29 degrees Sagittarius, so those of you Sagittarians born after December 18th can feel the start of this retrograde most. The retrograde ends at 13 degrees Sagittarius, so those of your Sagittarians born December 2nd - 8th can feel the end of the retrograde 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.
Suggested Reading: The Mercury, Venus and Mars Retrogrades page for general interpretations