Juneteenth, an annual holiday commemorating the end of slavery in the United States, has been celebrated by African-Americans since the late 1800s.

But in recent years, and particularly following nationwide protests over police brutality and the deaths of George Floyd, Breonna Taylor, Ahmaud Arbery and other African-Americans this year, there is a renewed interest in the day that celebrates freedom.

This year’s celebration may resonate in new ways, given the sweeping changes and widespread protests across the U.S. President Trump had planned to hold a campaign rally in Tulsa, Okla., on the holiday, June 19. But late Friday night Mr. Trump, bowing to pressure, announced that he would delay it for one day.

Here’s a brief guide to what you should know about Juneteenth.

What is Juneteenth?

On June 19, 1865, about two months after the Confederate Gen. Robert E. Lee surrendered at Appomattox, Va., Union Gen. Gordon Granger arrived in Galveston, Texas, to inform enslaved African-Americans of their freedom and that the Civil War had ended. General Granger’s announcement put into effect the Emancipation Proclamation, which had been issued more than two and a half years earlier on Jan. 1, 1863, by President Abraham Lincoln.

The holiday received its name by combining June and 19. The day is also sometimes called “Juneteenth Independence Day,” “Freedom Day” or “Emancipation Day.”

How is it celebrated?

The original celebration became an annual one, and it grew in popularity over the years with the addition of descendants, according to Juneteenth.com, which tracks celebrations. The day was celebrated by praying and bringing families together. In some celebrations on this day, men and women who had been enslaved, and their descendants, made an annual pilgrimage back to Galveston.

Celebrations reached new heights in 1872, when a group of African-American ministers and businessmen in Houston purchased 10 acres of land and created Emancipation Park. The space was intended to hold the city’s annual Juneteenth celebration.

Today, while some celebrations take place among families in backyards where food is an integral element, some cities, like Atlanta and Washington, hold larger events, like parades and festivals with residents, local businesses and more.

Galveston has remained a busy site for Juneteenth events over the years, said Douglas Matthews, who has helped coordinate them for more than two decades. He said the city usually has about 15 events, including parades, barbecues, musical performances and a beauty contest, beginning in the first week of June. Combined, the events draw about 10,000 people in total, he said.

“This year, there’s only probably about two or three events because of the coronavirus epidemic,” he said. “We’re just happy with the significance of our Juneteenth anniversary.”

Would it ever become a national holiday?

In 1980, Texas became the first state to designate Juneteenth as a holiday, though the recognition is largely symbolic. Since then, 45 other states and the District of Columbia have moved to officially recognize the day. In 2019, New Hampshire became the latest state to declare Juneteenth a state holiday.

This month, Twitter and Square, a mobile payment company, designated Juneteenth as a company holiday. Jack Dorsey, the chief executive and a founder of Twitter and Square, said the day was for “celebration, education, and connection.” Vox Media, Nike and the N.F.L. have also made similar announcements, joining others in making the celebration a paid day off.

Why has Juneteenth become so important this year?

Following the filmed killing of George Floyd, a 46-year-old African-American man who died in the custody of the Minneapolis Police last month, thousands of people around the U.S. have poured onto the streets in protest. Mr. Floyd’s name, as well as the names of Ms. Taylor, Mr. Arbery, David McAtee and others, have become rallying cries for change across the country, effectively re-energizing the Black Lives Matter movement.

That change has come in waves. In Minneapolis, officials banned the use of chokeholds and strangleholds by police, and said officers must intervene and report any use of unauthorized force.

Democrats in Congress unveiled sweeping legislation targeting misconduct and racial discrimination by the police. The bill is the most expansive intervention into policing that lawmakers have proposed in recent memory.

Companies across the business spectrum have voiced support of the Black Lives Matter movement and have either suspended or fired employees who mocked Mr. Floyd’s death or made racists remarks.

Mark Anthony Neal, an African-American studies scholar at Duke University, said there are some comparisons between the end of the Civil War to current unrest, adding that this moment feels like a “rupture.”

“The stakes are a little different,” Mr. Neal said. “Many African-Africans, black Americans, feels as though this is the first time in a long time that they have been heard in a way across the culture.”

“I think Juneteenth feels a little different now,” he said. “It’s an opportunity for folks to kind catch their breath about what has been this incredible pace of change and shifting that we’ve seen over the last couple of weeks.”

Why did the president’s plans to hold rally on June 19 in Tulsa, Okla., draw criticism?

For the first time since the coronavirus outbreak shut down most of the country, Mr. Trump will return to the campaign trail. He had planned a rally in Tulsa on June 19. In 1921, the city was the site of one of country’s worst episodes of racist violence, when white mobs attacked a wealthy black business district known as Black Wall Street. The mobs destroyed more than 1,200 homes and killed as many as 300 people.

The announcement drew ire from Mr. Trump’s critics. “This isn’t just a wink to white supremacists — he’s throwing them a welcome home party,” Senator Kamala Harris of California tweeted. In response to the backlash, Brad Parscale, Mr. Trump’s campaign manager, touted Mr. Trump’s “solid record of success” for African-Americans.

Asked in an interview with Fox News on Friday if the rally was set on June 19 on purpose, Mr. Trump said no. “The fact that I’m having a rally on that day, you can really think about that very positively as a celebration,” he said, adding, “It wasn’t done for that reason. But it’s an interesting date.”

Melanye Price, a political science professor at Prairie View A&M University, a historically black university in Texas, called the planned rally on Juneteenth a “slap in the face.”

“In the middle of the largest protests we have ever seen in this country about racial justice, he shows up in a town on a day that is meant to celebrate racial freedom and liberation, in a town where the black population was decimated by racial prejudice and white supremacy,” Ms. Price said. “Who doesn’t see this as a problem?”

Late Friday night, in a Twitter message just before midnight, the president announced that the rally would be held the following day instead.

“We had previously scheduled our #MAGA Rally in Tulsa, Oklahoma, for June 19th — a big deal,” he wrote. “Unfortunately, however, this would fall on the Juneteenth Holiday. Many of my African-American friends and supporters have reached out to suggest that we consider changing the date out of respect for this Holiday, and in observance of this important occasion and all that it represents. I have therefore decided to move our rally to Saturday, June 20th, in order to honor their requests.”

Celebrate Juneteenth by making cocktails with Toni Tipton-Martin, award-winning food journalist and nutritionist, and Veronica Chambers, senior editor, Special Projects for The New York Times. They’ll be talking about their writing and how they explore race, history, memory and meaning on and off the page. Thursday, June 18 at 6 P.M. E.T.

R.S.V.P. to join virtually.