It shouldn't happen to a dog -- having a birthday on Dec. 25 -- but it has. And guess which dog.

If you guessed Millie, the Bushes' best paw forward at the White House, you were right. But don't think the nation's top dog will be shortchanged when she turns 5 on Christmas Day. Assuming she receives the same generous attention as her editor at William Morrow & Co., she'll be inundated with rawhide bones, rubber squeakers, balls with bells and maybe even a pair of chewable Gorby and Raisa or George and Barbara bedroom slippers.

Ever since Morrow brought out "Millie's Book" in September, its vice president and senior editor Lisa Drew has been reaping the rewards of laughing "at all the right places," as Millie and her "dictationist," Barbara Bush, put it in their dedication to Drew.

Heading into its 13th consecutive week on the New York Times bestseller list, "Millie's Book" will hold firmly at No. 4 when the Dec. 23 list is released. Now the question is not so much whether Millie's is outselling Ronald Reagan's book, for which Simon and Schuster reportedly paid a $6 million advance (and which will drop from No. 5 to No. 8), but whether it will survive longer than Nancy Reagan's book, which dropped off the list last year after 14 weeks.

Meanwhile, Barbara Bush told reporters that her decision to have Millie spayed by a Columbia, S.C., veterinarian "had nothing to do with anything except that her doctor was sort of afraid to have her operated on unless she went to his best friend, who was a surgeon. It made sense."

An Army veterinarian who has been treating Millie with cortisone for a type of lupus affecting bones, muscles and nerves drove the English springer spaniel to the South Carolina clinic "on his own time -- we paid for it," according to Mrs. Bush.

Had Millie become pregnant, Mrs. Bush said, "it would have been very detrimental to her. As it is, at any moment she could go out of remission. We're not fooling ourselves about that. She's a miracle anyway and the doctors are all amazed that she is well."

According to Mrs. Bush, "only 45 dogs have ever been diagnosed as having lupus. On the other hand, I attribute that to the fact that a lot of people wouldn't have gone to the trouble this doctor has. He no sooner walked into the job than this happened."

Dorothy Bush LeBlond may be shy and unassuming in public, but behind the scenes on President Bush's South American tour, "George said she was 'good company, funny, cute and lively,' " Barbara Bush said last week of her only daughter and youngest child.

A last-minute substitute for the First Lady, who was suffering from a sinus infection, "Doro" LeBlond represented her mother at a couple of events during her father's five-nation trip. She also joined him in a tennis match against Argentine President Carlos Menem.

"She loved it," Mrs. Bush said of her daughter's assignment, "though after about three days on the road, she was anxious to get home."

And that's probably a good thing, since the First Lady has no plans to relinquish on a regular basis her role of First Traveling Companion when Bush goes abroad.

Said Mrs. Bush, laughing: "I told her, 'Don't get used to that. Don't get to like it.' "

That old gang of his has lost a few members -- among them his Kitchen Cabinet's William French Smith, Holmes Tuttle, Justin Dart and Alfred Bloomingdale -- in the decade since Ronald Reagan's big 70th-birthday party at the White House. But there are still plenty of other pals around to help the former president mark his 80th on Feb. 6 at the Beverly Hilton.

Very much involved in every aspect of the tribute is Nancy Reagan. Assisted by Lod Cook, Arco's CEO, who is chairing the party, and a 30-member committee that includes Joe and Barbara Allbritton, Marvin and Barbara Davis, Steve and Sabina Forbes, Charles and Mary Jane Wick, Erle and Marion Jorgensen, Lew and Edith Wasserman and Paul and Carol Laxalt, Mrs. Reagan has signed up Liza Minnelli for the gala show that night and good friend Merv Griffin as the master of ceremonies.

Some of the committee have pledged to buy tables at $25,000 a throw, according to Robert Higdon, finance director of the Ronald Reagan Presidential Foundation. Tickets on the next rung of philanthropy will sell for $1,000 each, and about 1,000 invitations to buy them will be mailed in the next few weeks.

Proceeds will go to the Reagan Library, currently under construction in the Simi Valley area of Ventura County, 45 miles north of Los Angeles. Now about 80 percent completed, the facility is scheduled for a November 1991 opening.