First of all, I would like to thank Ashley Marando in allowing me to use her excellent photograph of the Sun.
Mercury will pass between the Earth and the Sun on May 9 2016. A telescope with a SOLAR FILTER is required to see this celestial event. Mercury is too small for the eye to see without a telescope. The angular diameter of this planet on May 9th is 12 arc seconds. The smallest the human eye can see is around 60 seconds of arc so a telescope (with a solar filter) is definitely required. Parts of eastern US will see the entire transit which will last seven and a half hours starting soon after sunrise on the east coast. The next transit will be on November 11, 2019.
The transit of Mercury happens 13 or 14 times a century -- either in May or November.