Mayor Bill de Blasio was officially sworn in for his second term during a pre-inaugural reception at Gracie Mansion on Sunday.

The relatively small affair, which came a day before de Blasio’s public swearing-in ceremony and inauguration at City Hall, was officiated by City Clerk Michael McSweeney.

De Blasio told those gathered Sunday he plans to talk about his aspiration that the Big Apple becomes the “fairest big city in America” during his Monday inaugural.

“We’re on the right path in this city,” he said. “We’ve proven in the last four years how much can change and how quickly it can change.”

Vermont Sen. Bernie Sanders will officiate the second swearing-in on Monday at a noon ceremony that’s expected to cost $200,000, a portion of which is expected to be paid for through private funds.

De Blasio did not endorse Sanders during his 2016 presidential run, instead famously waffling for months on his endorsement of Hillary Clinton.

President Bill Clinton swore de Blasio in during his first inauguration on Jan. 1, 2014.

Officials expect as many as 1,500 people for Monday’s event — a scaled-down ceremony compared to 2014.

With temperatures expected to be about 18 degrees around noon, de Blasio warned those coming had better bundle up.

“I hope you’re hardy enough to join us,” de Blasio said Sunday. “I would suggest that you wear every type of clothing available. You should layer things that have never been layered before.”