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 February 8th and ends April 6th, lasting eight weeks. The first storm period will be from February 19th - 28th, and the second will be from March 12th - 26th. The exact retrograde begins February 23rd at 4:41AM ET and ends March 17th at 4:03PM ET.
The water signs are taking their turn with the Mercury retrogrades, and this retrograde will occur entirely in the sign of Pisces. What does Pisces rule? Well, let's start with the general fact that this is a water sign, so our emotions are all askew. We too closely identify with things and become emotional in situations where it's not warranted, and it's harder to calm us down. This is especially true in Pisces, which rules the emotional train wreck (as a Pisces Moon, I know that well). Since it's a Mercury retrograde and Mercury rules communications, we're too sensitive with the words spoken to us, and too insensitive with the words we speak to others.
Pisces rules karma and emotional baggage, and that could be coming back to bite us as a whole. Countries/societies may see things from the past come back to haunt them. The things we thought were dead are suddenly dancing in our faces and delighting in the torment. Secrets could come out, dirty little secrets that we're ashamed of. We try to hide as much as possible, but don't succeed at all.
Pisces also rules fantasy and imagination, where the dark side is illusions and delusions. We may have a difficult time seeing the reality of situations, not wanting to face the truth and believing only the lies we tell ourselves to make ourselves feel better. If you find yourself muttering about how delusional everyone seems to be, you'll know why.
Pisces rules water (so we could see a prevalent water issue), feet, music (a music scandal?), as well as addictions and escapism (maybe someone high profile enters rehab, falls off of the wagon, or we're all drinking like fishes to deal with this retrograde!).
The turn retrograde begins at 19 degrees Pisces, so those of you Pisces born March 7th - 11th may feel the turn retrograde most. The turn direct occurs at 5 degrees Pisces, so those of you Pisces born February 22nd - 26th 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.
Mercury Retrograde in Sagittarius: December 2017
November 2017 New & Full Moons
December 2017 New & Full Moons
Jupiter in Scorpio: October 2017 - November 2018
Saturn in Capricorn: December 2017 - March 2020, July - December 2020
Saturn in Sagittarius: December 2014 - June 2015, September 2015 - December 2017
Saturn in Sagittarius for Each Sign