As spring turned to summer this week, extreme heat caught much of New York City by surprise, sending concerned residents to prepare for rising temperatures earlier than usual, while various city agencies, civic organizations and businesses have also been adjusting their seasonal calendars.

As climate change extends the traditional summer and batters the city with spontaneous rain storms, many New Yorkers are discovering that they are not quite in sync with the weather anymore and have to rethink how to adapt to the passing months.

Look no further than this week: The city has already activated its first heat emergency plan of the year, but school remains in session, pools have yet to open and public beaches are still staffing up with lifeguards.

For the most part, New Yorkers made modest adjustments to their schedules, postponing outings or being cautious if they pressed ahead, exercising and running errands, for example, during the morning and the evening — the coolest parts of the day.

“There’s always some natural variability in weather and climate, but climate change is loading the dice so that the calendars we relied on in the past are becoming a losing proposition,” said Radley Horton, a professor at Columbia University’s Climate School.

Extreme heat is striking both earlier and later in the year because of global warming, Dr. Horton said. This means that all days are warming up, not just summer days, he explained, adding that “there is also concern that poorly understood surprises with climate change, like very early snow melt, which leads to very early drying of soils, could be making hot days even hotter.”

The New York City Emergency Management Department studied extreme heat throughout the year to prepare for this summer, said Ashleigh Holmes, a spokeswoman for the agency. Because of the advance work, the department was able to update its cooling center map three weeks before the early heat wave, she added.

But the calendar worked against those efforts this week when public libraries, which make up roughly a third of all cooling centers, were closed for the Juneteenth holiday on Wednesday.

“It’s important to emphasize that there are numerous ways to stay cool in New York City, including over 300 cooling centers that are not libraries, and there have been holidays in the past when libraries have not been opened,” said Aries Dela Cruz, the executive director of public information at the Emergency Management Department.

One of those other ways to stay cool is the Home Energy Assistance Program, a state program that gives away air-conditioners and has been up and running since April. Anticipating a hotter, earlier summer, officials introduced the program two weeks earlier this year than last year, a state spokesman said.

When Rudy Thomas, 52, a live-in home health aide in Coney Island, heard this week about the impending heat and the air-conditioner program, he raced to track down the paperwork to apply.

But on Tuesday, when Mr. Thomas showed up at the Department of Social Services, which helps to administer the program, there was no one to help him. He waited for three hours and then left, calling the errand a waste of time.

A spokesman for the department said he would look into what happened with Mr. Thomas’s attempt to file an application and that there was a drop box at the office for such requests.

For those who must work outside, like construction workers, the heat can be particularly punishing. For now, their schedules have not changed as much as been tweaked.

“Many unions negotiate breaks and other safety measures for temperature-related issues,” said Mario Cilento, the president of New York State AFL-CIO, which represents 3,000 unions — including building trades — across the state. He added that more regulation was needed to protect workers in the era of climate change.

Delivery workers usually see their schedules become more demanding during extreme weather, said Ligia Guallpa, executive director of the Workers Justice Project, a nonprofit group that seeks to improve the labor conditions of low-wage, immigrant workers.

When New Yorkers stay indoors because of soaring temperatures, this means heavier workloads for delivery workers who, as independent contractors, have no means to turn down jobs without fear of losing future work, she explained.

The city has started a pilot program to hand out what it calls “cool kits,” including cooling towels, cold packs, water and sunscreen, to day and delivery workers, said Ms. Holmes of the city’s Emergency Management Department.

New York State United Teachers, the state affiliate of the American Federation of Teachers union, is not yet pushing to change the school calendar. But the group, which has 700,000 members, has backed a recently approved law that aims to protect both children and teachers from high temperatures. The bill proposed that schools be evacuated if the temperature reaches 88 degrees.

Extreme weather can also affect the city’s cultural calendar, which features many outdoor events this time of year. Little Island, a public park on the Hudson River, has canceled two performances because of extreme heat since it started curating live concerts in 2021. During its first season, there were daytime shows, but because of the heat, all performances shifted to the evening, now starting as late as 8:30 p.m., a spokeswoman said.

New York Road Runners, a running organization that has canceled three races in the past two years because of extreme weather, posts guidelines for die-hards, while its Striders program, for older walkers is in constant consultation with its safety team, which recommends that participants switch to indoor programs or cancel when needed, a spokeswoman said.

Some picnickers in Central Park, however, didn’t seem interested in changing their plans at all, said Wendy Weston, the owner of Perfect Picnic, which caters picnics (although one client changed her start time this week from noon to 10 a.m.) “I think people are just really excited to be outside,” she said.