Mercury turns retrograde, apparently moving backwards against the stars, three or four times a year for about three weeks at a time, an effect of the Earth overtaking the faster inner planet on the inside of its orbit. The classical tradition treated retrogradation as a weakening or confusing of a planet's action; the modern popular doctrine that Mercury retrograde specifically disrupts travel, contracts and electronics is a twentieth-century elaboration and should be dated as such. The god behind the planet is the Greek Hermes and the Roman Mercury, messenger, guide of souls to the underworld, patron of merchants and of thieves, the divinity of boundaries and of what crosses them; the Mesopotamian planet was assigned to Nabu, god of scribes. The identification of the planet with Thoth and with Hermes Trismegistus is what gives the whole Hermetic literature its name.
The three or four retrogrades a year follow from a single figure. Mercury's synodic period, the interval between successive conjunctions with the Sun as seen from Earth, is about 116 days, so the cycle repeats a little over three times in a year and the retrograde arc falls at the middle of each. The same geometry produces a second and older doctrine. Mercury never appears more than about 28 degrees from the Sun and often much less, so it is never more than a sign away from it and spends a large part of every year within the classical distance for combustion, which the tradition read as a debility in its own right. A Mercury free of the Sun's beams is the exception, not the rule.