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No tables copied from another website: this page tracks Mercury’s actual apparent motion with the site’s ephemeris and finds the days it turns around. The answer below is computed for the moment you open the page.
Dates are the station days — when Mercury appears to stop and reverse — computed from the JPL approximate elements, the same engine whose oppositions land on the published days. Expect agreement with printed calendars within about a day.
Mercury never actually reverses. Three or four times a year, Earth — moving slower on its wider orbit — is overtaken in the inner lane: Mercury, lapping us, appears against the fixed stars to slow, stop, and loop backward for about three weeks, then stop and resume. It is the same illusion as a train you pass seeming to drift backward, raised to the scale of the solar system. You can watch it happen, honestly, in this site's Sky Simulator: set the speed to a day per second and look at Mercury against the stars.
The checker works the way an astronomer would: it measures the rate of change of Mercury's geocentric longitude — where the planet sits against the zodiac. When that rate turns negative, Mercury is retrograde by definition. The periods below come from scanning that rate day by day across this year and next and bracketing the two station days of each loop.
What the tradition makes of retrograde Mercury — tangled communications, travel hiccups, a season for review rather than launch — is astrology's oldest running commentary on the phenomenon; the astronomy above is exact either way.
Three, occasionally four, times a year, for roughly three weeks each time — about 19% of the year in total. If it feels constant, that is partly the shadow periods astrologers add before and after, and partly Mercury’s talent for timing.
The station is an instant, and sources round it to a day in different time zones. A one-day disagreement is the same event wearing two calendars.
Yes — note Mercury’s position against nearby stars over successive evenings with binoculars during a loop, or run the Sky Simulator here at a day per second and watch the loop draw itself.