Mercury orbits the Sun in about eighty-eight days, but because of its retrograde motion it takes about a year to traverse the entire zodiac. Mercury normally takes less than three weeks to move through a sign when its motion is direct. It will take eight to ten weeks in a sign when it is retrograde. Those five or more extra weeks Mercury retrograde spends in one of your natal houses means that Mercury's energy is directed at the affairs associated with that arena of life as represented in your natal birthchart.
Mercury rules mental processes and all forms of communication including travel, speaking, and writing. This means that messages, letters, books, magazines, contracts, the signing of documents, neighbors, the neighboring environment, commuter-type travel, and transportation facilities all fall under Mercury's rulership.
Therefore, when Mercury turns retrograde by transit, think in terms of the possibility for communication breakdown. Mercury is the Director of the Communication Department and while retrograde, is on sabbatical for about three weeks. The details normally handled by the Communication Department aren't being tended to as carefully as they are when Mercury is direct. All types of communication have the potential for becoming confused, delayed or misunderstood. There can be confusion, changing of plans, indecisiveness, mail-slowdowns, lost letters, transportation problems, electronic equipment breakdowns, and failures in negotiations--all the more if Mercury's retrograde involves the sign Gemini or your natal third house. For these reasons, it is felt that the Mercury retrograde period is not a good time for signing contracts, scheduling negotiations, or launching new projects. It's a good time for planning and organizing anything that will be finalized after Mercury has returned to direct motion.