On 31st July 2016, Mercury moves into one of its home signs, Virgo ( the other home sign being Gemini).
Mercury in Virgo indicates that a person has an analytical, bright and sharp mind. It lends to a talent with analysis, detail and precision, it can also indicate an inclination to worry, particularly about health.
As ever though, one has to look at the overall natal chart to see the bigger picture of this placement. In some cases it maybe less prevalent depending on other indicators in the chart such as the aspects, houses and signs. Astrology is never a one-size-fits-all matter.
Mercury in Virgo adds a practicality since Virgo is an earth signs so likes to harvest and produce.
Madame Blavatsky, who is the subject in Chapter 3 of my forthcoming book Compass of The Stars, had this placement in her birth chart.