“OK, today is Friday! Let’s rock and roll!” yelled the bartender at McGarry’s pub in Midtown, just after the tavern opened at 11 a.m. this week.

In minutes, hard-drinking construction workers at the booming Hudson Yards site swarmed in to pound beers, down shots and gulp mixed drinks before heading back to work.

Post reporters observed on the past two Fridays workers getting blitzed at the Ninth Avenue watering hole during their hour-long meal breaks, then returning to a bustling construction zone that features multiple fast-rising skyscrapers and several hundred hardhats.

On July 6, 22 workers crammed into the bar by noon. Four men standing in a row threw back three Bud Lights, each in quick succession, before they left. Others drank double Stoli and sodas in pint glasses.

The bartender — who said he’s worked at McGarry’s for 12 years — knew many of the workers by name and didn’t need to be told what they wanted to drink.

One construction worker, a man in his mid-30s, entered the bar at 11:30 a.m. and left by 12:15 p.m. In that time, he tossed back five Jameson shots and four Stoli and sodas. He did not eat but occasionally went out back for smoke breaks.

When he left, he stumbled and swayed while walking back to his job site at 500 West 33rd Street.

This Friday, a construction worker in his late 20s wearing a T-shirt and jeans entered the bar around 11:20 a.m. and sipped a beer as the bartender poured him what appeared to be a glass of whiskey.

Then three men in jeans entered, and they too ordered beers.

“Happy Friday!” shouted one, and the four clanked bottles of Guinness. Minutes later, they were throwing back shots.

By noon, there were 20 construction men and women jammed in the bar, all drinking beer or doing shots. Several ordered burgers, fries, nachos or chicken tenders, but many did not eat.

“Liquid lunch?” one worker teased another.

At 12:15 p.m., the man in the T-shirt and two others exited the bar and walked one block west on 33rd Street, where they entered an area with a “Construction workers only” sign at 33rd Street and 10th Avenue.

Heavy lunchtime drinking has been a problem in New York for many years, including in 2009, when the Port Authority canned hardhats after The Post exposed World Trade Center construction crews getting loaded before returning to work.

“This needs to stop,” said Brian Sampson, New York president of the Associated Builders & Contractors.

“When you’re in New York City and you’re building vertically you need to have the confidence that your co-workers are not under the influence of drugs or alcohol.”

Joanna Rose, a spokeswoman for Hudson Yards developer The Related Companies, said: “We have a zero-tolerance policy for alcohol or drugs being used by personnel working on-site.”