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 December 19th and ends February 14th. The exact retrograde begins January 5th at 8:06AM ET and ends January 25th at 4:50PM ET. The first storm period is from January 2nd - 9th, and the second storm period is from January 21st - February 1st.
This Mercury retrograde begins early in the sign of Aquarius, and we can feel disconnected to one another, have difficulty getting along with each other, and have trouble with groups. Large organizations and charities can come under fire. We can question the future, unsure of what’s to come, and we can feel that things won’t work out the way we want them to. Friendships can be strained, tech can go wild, and we’re ultra rebellious. We can be a lot more rebellion with Mercury retrograde in Aquarius.
This retrograde won’t last long in Aquarius, quickly moving back into earth sign Capricorn on January 8th. This is where the bulk of the action happens, and where the bulk of the impact will be. Capricorn rules governments and politicians, so they tend to not behave so great with Mercury retrograde in Capricorn, and we can see more anger with them (like there isn’t enough as it is!). Capricorn rules authority figures and our elders (parents, bosses, mentors), and we can rebel against them as well.
Capricorn rules restriction and limitation, and we can feel others are stifling us too much, or we choose to hold ourselves back too much. Capricorn also rules responsibility, maturity, and practicality, all of which can be an issues during the retrograde, and adults act like half-brained teenagers, we’re pushing back against the rules, or we can be so serious and adult that we’re dull and lifeless. Try to focus on some Capricorn positives, like recommitting to old goals, reconnecting with old elders, or reviewing plans with a practical eye.
This Mercury retrograde starts square (a hard aspect) Mars, freshly in Scorpio (exact on the 5th at 7AM, just before the retrograde starts that day). Then the retrograde ends conjunct (in the same spot as) Pluto in Capricorn (exact on the 22nd and 30th). So the start and end of this retrograde will be far, far from easy! We’ll have some seriously contentious energy to find a way to deal with, and we shouldn’t be surprised if we’re seeing a lot of people exploding, a lot of anger and rage, a lot of misdirected energy as this retrograde begins and ends. Try to give yourself something to keep you busy at the start, and work on transforming something at the end.
Other than that, this Mercury retrograde will conjunct the Sun on January 14th, bringing more emphasis and focus on the retrograde. It can also show the start of something that reaches completion in about a month. It's helped by Mercury retrograde trine Jupiter that day as well.
Mercury retrograde also squares Uranus on January 20th, which can make us avoid the unconventional and different and uncomfortable with change, or we go too far with change and make things wonky. This is a wonky retrograde anyway! So be sure to refrain from big changes, and try to manage these difficult aspects as best as you can.
The turn retrograde begins at 1 degree Aquarius, so those of you Aquarians born before January 24th may feel the turn retrograde most. The turn direct occurs at 14 degrees Capricorn, so those of you Capricorns born January 4th - 6th 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.
Suggested Reading: The Mercury, Venus and Mars Retrogrades page for general interpretations