☿Mercury retrograde
Three times a year, for about three weeks, Mercury appears to move backward across the sky (an optical illusion created by our view from Earth). In astrology it is known as a period for revisiting themes of communication, correspondence, travel and technology.
In the 'shadow' periods before and after the retrograde, the planet crosses the same degrees it will cover while retrograde, and its themes can be felt during those days too.
♀︎Venus retrograde
About every 18 months, Venus appears retrograde for roughly forty days. In astrology it is read as a time for reflecting on love, values, aesthetics and past relationships.
The shadow periods are the weeks before and after, when Venus travels across the degrees it will revisit during the retrograde.
♂︎Mars retrograde
About every two years, Mars appears retrograde for around two months. In astrology it is described as a time when energy, goals and motivation turn inward and strategy gets rethought.
Mars has long shadow periods, covering the weeks around its entry into and exit from the retrograde.