At street fairs, in baseball parks, even on the subway, people reveled in the familiar Saturday as the Midwest and Northeast almost fully recovered from the worst power outage in U.S. history.

While cities from New York to Detroit slipped back into the pace of a summer weekend, investigators turned their attention to three transmission lines in Ohio that may have sparked Thursday’s blackout.

Day One A.B. — after blackout — arrived in the nation’s largest city with the subway system back on track, once again zipping New Yorkers from the Battery up to the Bronx.

Power was officially restored to the entire city at 9:03 p.m. Friday, nearly 29 hours after it went out almost simultaneously in eight states and Canada. The subways started rolling just after midnight Saturday.

In Ohio, Cleveland’s power was fully restored Friday, in time for both baseball’s Cleveland Indians and the NFL’s Browns to play at home. The lingering problem was that city residents had to boil water before drinking or cooking with it.

People in Detroit, where water pressure was low, were under a similar order, though power was restored to nearly all of Michigan’s 2.3 million customers Saturday. Energy officials called the situation tenuous and said it was critical for people to conserve electricity to avoid rolling blackouts.

“If customers will cooperate one more day, I think we’ll be able to avoid rolling blackouts,” DTE Energy chairman and chief executive Anthony Earley said Saturday afternoon.

The blackout occurred at 3:11 p.m. CDT Thursday, creating instant chaos in the eastern United States and the Canadian province of Ontario.

Officials there struggled Saturday to restore stable power throughout the province, but warned it could take days before everything is back to normal.

In Manhattan’s Midtown, hundreds wandered about on a sticky summer day at a street fair — a typical, and welcome, scene after two days without power.

There were still overflowing garbage cans scattered around Manhattan, but sanitation crews were working overtime through the weekend. Tons of trash had been piled on sidewalks as New Yorkers emptied their refrigerators of spoiled food.

The only places in New York still suffering from a major blackout hangover were the three airports: Kennedy International, LaGuardia and Newark Liberty. With several hundred flights canceled on Friday, “the airlines are still trying to get their schedules back in order,” said Port Authority spokesman Steve Coleman.

The City Council finance office estimated the blackout cost the city up to $750 million in lost revenue, up to $40 million in lost tax revenue and up to $10 million in overtime pay for the first 24 hours after electricity went out.