On May 9th a transit of Mercury occurs when the planet crosses the face of the Sun for the first since 2006. The event is visible (at least partially) from most parts of the World. It begins at 11:12 UT, ends at 18:42 UT with the midpoint occurring at 14:58 UT. The transit lasts for 7.5 hours in total and observers will notice a tiny 10 arc second diameter dark disk moving slowly across the solar face.
If you're living in eastern North America, Western Europe, Northwestern Africa and much of South America the complete transit is visible. In western North America and southern parts of South America it will have already started at sunrise whereas for those living in most of Africa and Asia the transit will still be in progress at sunset. Finally folks in Australia, New Zealand, Japan and other parts of eastern Asia will have to watch it online.
You do need a telescope to observe the event. As always it's extremely important to exercise due care and use only safe observing techniques when looking at the Sun. Never look at the Sun directly it's incredibly dangerous and will cause permanent eye damage. Observe the transit with either specialised solar telescopes or normal scopes fitted with approved filters over the main lens or mirror of the instrument. It's not safe to use filters at the eyepiece end. Alternatively, you can project the solar disk onto a shielded white card ensuring that all finderscopes are removed or safely covered.
The most interesting aspects are when Mercury enters and exits the Sun's limb. During ingress you will see a small dent just south of the eastern edge of the Sun. Mercury will then move slowly across the face of the Sun. Since the disk is small it can be confused for a sunspot although Mercury appears precisely round and lacks a penumbra. Seven and a half hours later the transit will end at the southwestern limb. Watch out for the black drop effect. This illusionary effect gives the appearance that the planet is still connected to the darkness by a thin black line as it leaves.
Transits of Mercury aren't as dramatic as those of Venus. However they do occur more often with the next one visible on November 11, 2019. For comparison, the next Venus transit doesn't occur until 2117!