First up, on May 5: The biggest full moon of the year will occur this weekend! Also known as a "supermoon", this is the time when the moon is at its perigee (when it's the closest to the earth in its elliptical orbit), and also when it's full. What this means is that the moon is going to be bigger and brighter than average, and may give some spectacular views as it's rising!
the march 2011 supermoon as seen in washington (credit: tim mccord @ space.com/)
On May 20, an Annular Solar Eclipse (when a ring of sunlight will be visible around the eclipsing moon) will be visible for China and the Western US, although those in Northern California will have the best view. NASA has an interactive map, which allows you to view eclipse times in your location. The next full solar eclipse visible in the US won't be until 2017!
an example of an annular eclipse (credit: NOAA/SEC)
And finally, saving the best for last, on June 5 in the US (or June 6 on the other side of the world), a very special event: the Transit of Venus! This is the last time this will happen in our lifetimes, so it's a definite can't-miss event. A Transit of Venus occurs when "Venus passes directly between earth and the sun" and "we see the distant planet as a small dot gliding slowly across the face of the sun". The last Transit was in 2004, but the next one won't be until 2117.
the 2004 transit of venus, seen in grand rapids, mi (credit: leapsecond.com)