If you just pay attention to US equities, you probably noticed the big 200 point Dow drop on Friday, but mostly it was a fairly uneventful month.
But for world markets there were huge twists and turns that will likely be echoing for a long time.
Here are the big three.

The Real Rate Rise

Despite the post-crisis economic recovery, real interest rates in the US have been on an uninterrupted steady grind lower. Real interest rates adjust nominal interest rates for inflation to give you a sense.
For the first time since the crisis, we're seeing what looks like a reversal of this trend, amid all kinds of talk about economic recovery and the Fed "tapering" the pace of QE.
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FRED

ZARMAGEDDON

With real interest rates rising in the US, the dollar has strengthened nicely. That's one factor that's crushing the currencies of emerging markets, which had been gigantic winners. Coupled with the weakness in commodity prices (and other factors) the South African rand got absolutely poleaxed in May.
Here's a chart showing the South African Rand (ZAR) vs. the US Dollar (USD) over the last month.

The Japan Crash

Starting last autumn, Japan's Nikkei was the hottest market in the world, thanks to the experiment known as Abenomics (which involves extraordinary monetary easing).
After a virtually uninterrupted upward surge, the Nikkei got destroyed over the last week or so, falling about 15% from the peak.
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So there you go.
The turn in rates, the crushage of emerging markets, and the reversal in Japan. May was a big month.
Meanwhile, there's a ton of economic data coming out this week that everyone will be watching.