Was fairly common before 2015 or so. May 4-10, 2003 had four, so did May 22-30, 2004 and three between May 22 and June 5, 2008. In fact, 4-6 per year was the general expectation from the 1990s through the early 2010s (2010 had six including one in October, 2011 had five including back-to-back on April 26-27); except in real dud years like 2000 and 2009. Then dud years started becoming the rule rather than the exception.