Mercury is one of only two planets in the Solar System that can be observed passing in front of the
surface of the Sun from Earth. The other planet is Venus.
This is because both Mercury and Venus are positioned in between Earth
and the Sun. All of the
other planets are behind Earth in order from the Sun. The occurence of a planet passing
in front of the Sun is called a transit.
Although Mercury orbits the Sun every 88 days, it is actually quite rare for it to pass in front of the Sun when viewed from Earth. The Sun is huge, but when viewed from Earth is relatively small in the sky. Mercury is smaller still, so lining up Mercury in front of the Sun doesn't happen very often. The next time there will be a transit of Mercury across the Sun will be in November 2019. Then there will be a wait of 13 years for the next one, which will take place in November 2032. The most recent transits of Mercury in front of the Sun took place in May 2016, November 2006, May 2003, November 1999 and November 1993
Notice a pattern forming with the months when the transits take place? They only take place in May or November! At least from Earth they do. Over on Mars, it is also possible to observe Mercury transitting the Sun. In fact, it happens much more regularly. Of course, no human being has ever been to Mars, so the only observations we have are from unmanned rovers that have landed on the planet. So far only the Curiosity rover has done this, making its first observation on 3rd June 2014. This is the first time a planet has ever been observed transitting the Sun from a planet other than Earth.