Even beyond talk of a "June swoon," an unusual number of major events makes this month a critical time for markets.
From this week's European Central Bank meeting, to OPEC's Friday meeting and the G-7 over the weekend, the month is packed with events—not least of which is the Fed's June 16 and 17 meeting. Greece has a payment due to the IMF on Friday, and that has added a level of tension to markets this week.
"Clearly, the resolution in Greece and how that risk resolves itself is the biggest near-term risk factor that has us concerned," said Joseph Lupton, senior global economist at JPMorgan. "It has the potential to do the most damage."