Seersucker Thursday! Senators pose for their annual photo in the Capitol’s Senate Reception Room. (Bill Clark/ Roll Call via AP)

By the time obsessive C-SPAN viewers spotted their third senator in striped pastel cotton Thursday, a mild confusion set in. Coincidence? No — conspiracy! It was Seersucker Thursday, the Senate’s annual bipartisan celebration of southern summer style.

How do senators know when to dress up? Does a memo go around? Staffers seemed just as perplexed, so we went straight to the man who started the tradition in the late ’90s, former Sen. Trent Lott.

Sen. Kay Hagan, in seersucker on C-SPAN. (C-SPAN)

“Sen. [Mike] Lee had one. I didn’t think being from Utah he’d have seersucker, but he did. Amy Klobuchar had a nice-looking one. Kirsten Gillibrand looked really stunning in her seersucker.” Lott sounded pleased with the results. “I’m not involved,” though, he said, “other than to harass Mitch to make sure they do it.”