Get ready for publishing’s ultimate blockbuster sequel: Harry Potter VI.

Setting the stage for another round of midnight bookstore parties and marathon readings into the morning, the penultimate novel in J.K. Rowling’s mega-selling series, “Harry Potter and the Half-Blood Prince,” will go on sale 12:01 a.m. on July 16 in America, Britain, and four other countries, publishers said Tuesday.

“I know you all expected this to happen on Christmas Day, but I was sure that those of you who celebrate Christmas have better things to do on the day itself than fight your way into my study,” the British author wrote in a message posted on her Web site, “whereas those of you who DON’T celebrate Christmas would definitely prefer not to wait until the 25th.”

Ms. Rowling, 39, noted that while she is pregnant with her third child, she has had the time “needed to tinker with the manuscript to my satisfaction and I am as happy as I have ever been with the end result. I only hope you feel it was worth the wait when you finally read it.”

The book will also be published July 16 in Canada, Australia, New Zealand, and South Africa. With the previous works available in 62 languages, many, many other countries are sure to follow.

“J.K. Rowling has written a brilliant story that will dazzle her fans in a marvelous book that takes the series to yet greater heights,” said a joint announcement by Nigel Newton, chief executive of Bloomsbury Publishing in England, and Barbara Marcus, president of Scholastic Children’s Books in America.

The 2005 publishing date means the world will be spared the seemingly interminable three-year wait between Potter IV, “Harry Potter and the Goblet of Fire,” and Potter V, “Harry Potter and the Order of the Phoenix,” which came out in summer 2003.

The news should be celebrated by Rowling’s millions of fans and by the struggling publishing industry. Competition for sales began almost immediately, with Amazon.com and the superstore chains Barnes & Noble, Inc. and Borders Group, Inc. announcing 40 percent discounts on the book’s $29.99 suggested retail price. By Tuesday, “Half-Blood Prince” was already no. 1 on Barnes &Noble.com’s bestseller list and was in the top 10 on Amazon.com.

“Sales from the last Harry Potter book grossed as much as a major Hollywood movie in its first week of release,” said the chief executive officer of Barnes & Noble, Steve Riggio, in a statement yesterday.

“We expect this next book in the series to make publishing history once again. We’ve already collected 500,000 e-mail requests from our customers waiting to be notified of the date of the next book so this announcement is very welcome news for readers everywhere.”

The retail price is the same as for Potter V, even though Ms. Rowling has strongly hinted she will not approach the industrial-sized 870 pages of “Order of the Phoenix.” In an earlier message on her Web site, she stated, “According to the plan for book six, it will be quite a bit shorter than ‘Order of the Phoenix’. I am not going to swear on my children’s lives that that is going to be the case, but I am 99% certain of it.”

About 260 million copies of the fantasy series, which debuted in 1997, have sold worldwide, and “Order of the Phoenix” sold an astonishing 5 million copies in America alone within 24 hours of publication, generating as much money as the top Hollywood release of that weekend, “The Hulk.”

Sales have remained phenomenal even as Ms. Rowling’s books have grown longer and darker, reflecting the boy wizard’s maturation into adolescence. The first three Potter books have been made into hit movies and inspired countless Potter paraphernalia, including candy, cakes, capes, and toys.

For months, Ms. Rowling has been revealing details of her new book, naming the titles of three chapters – Chapter 2: Spinners End; Chapter 6: Draco’s Detour; Chapter 14: Felix Felicis – and even confiding that one of her characters will not survive, although she refused to say who. Mr. Potter himself is safe, at least for now. Ms. Rowling has said her teenage hero will survive until the seventh and final book in the series, but has refused to say whether he will reach adulthood.