It is the fate of rookie prosecutors to work a holiday in the city’s judicial coal mines. So New Year’s Day found new Manhattan DA Cy Vance watching some arraignments and shaking some hands.

“It’s good to be back,” said the man who served as a prosecutor in the mid-1980s under Robert Morgenthau, his legendary predecessor and mentor.

“I’ve spent many weeks in arraignments,” he said after a half-hour of watching 2010’s first defendants get processed. “But the nature of the cases is a little different now.”

Cases in point: ID theft, Internet fraud, texting harassment, and the unlicensed vending of New Year’s tchotchkes, a nuisance crime seldom prosecuted back in the day.

Vance rounded out Day One by meeting staff, visiting the DA’s complaint-drafting bureau and attending Mayor Bloomberg’s third swearing-in.