The day authorities arrested Denise Williams from her fifth-floor office at Florida State University was her daughter Anslee’s birthday.

Her only child was turning 19, and the blond accountant had raised her mostly by herself since her husband, Mike Williams, disappeared on a solo duck hunting trip Dec. 16, 2000, at Lake Seminole in Jackson County, Florida.

After he vanished, search efforts persisted for 44 days. But with the 31-year-old real estate appraiser’s body still missing, authorities speculated that he likely drowned in a boating mishap and then was eaten by alligators.

The gator theory, however, never sat well with Mike’s family, particularly his mother, Cheryl Williams. She couldn’t fight the nagging feeling that something more sinister happened to her son, according to friends.

Nearly 17 years later, a bombshell revelation upended the case, turning the probe into a murder investigation and securing Denise’s arrest in May on a grand jury indictment.

Denise, now 48, faces trial this week for allegedly hatching a plot with Mike’s childhood friend Brian Winchester to murder her husband so they could be together.

Prosecutors contend Winchester helped write a $1 million life insurance policy six months before Mike disappeared, and later married Denise, with whom he’d allegedly had a three-year affair while his best friend was alive.

The lithe widow was charged with murder, fraud, conspiracy, and accessory after the fact — all of which carry a sentence of life in prison.

But friends rejected the idea that Denise would have had any involvement in Mike’s homicide.

“As long as I have known Denise, she has always talked about Mike with a lot of love and respect,” friend Becky Maas told The Post. “She saw her future with him and that was taken from her.”

Suspicion turned to Denise and Winchester after their 2005 nuptials, according to local residents. But even when the pair ultimately divorced in 2016, authorities were not much closer to finding out what happened to the missing dad.

The investigation only came into sharp focus with a shocking confession last year from an unexpected source: the man who claims he shot Mike dead, Brian Winchester.

High school sweethearts

Denise and Mike were high school sweethearts at North Florida Christian School in Tallahassee, and most details from their early lives to the couple’s college days at Florida State are interchangeable.

Both were active in extracurriculars throughout high school. Mike was a promising football player and voted “best personality,” while his future wife — known then by her maiden name, Denise Merrell — cheered for the North Florida Eagles and landed the title of “best dressed.”

Former classmates said Mike was well respected at the private school, where he excelled in academics and served as student council president.

“Everyone just liked him because he was just a very genuine soul,” said childhood friend Patrick Shiflett.

It wasn’t unlike Mike to step into the role of protective older brother, he said.

“If he thought anyone was picking on you, he would stick up for you,” Shiflett told The Post. “Things can happen on the school bus, but he didn’t tolerate that. He was always there to protect me.”

Mike and Denise graduated high school in 1988 alongside classmates Brian Winchester and his future bride, Kathy Aldridge. The two couples remained close as they each got married in 1994 and had children.

Winchester, who was employed at a financial services firm, would later admit to authorities that he began an affair with Denise in 1997, according to court records.

“I had a good wife, I had a kid and I had Denise on the side. This is messed-up thinking, but in my mind, I had it pretty good,” Winchester allegedly told investigators in an audio recording The Post obtained. “Denise and Mike, on the other hand, they were at each other’s throats and she had 2 million reasons for this to happen.”

‘Two million reasons for this to happen’

While Denise was unhappy in her marriage, divorce was never an option, according to Winchester, who claims she was image-obsessed and concerned about losing custody of her daughter.

The Williamses were approaching their sixth anniversary, Winchester said, and the young mom dreaded an upcoming couples retreat planned for Dec. 16, 2000. Mike kept pressing the topic of conceiving another child, according to allegations in court records, and his wife wasn’t on board.

For 10 months, Winchester and Denise allegedly considered different scenarios that would get his wife and Mike out of the picture — including one in which both married couples set out on a boat that would capsize, prosecutors said.

However, it wasn’t until Denise’s anniversary trip loomed that their plot to kill Mike was actualized, according to allegations in court records.

She wanted a scenario in her mind where it wasn’t a murder — it was an accident.

“She wanted it to all be on me and not on her, and she wanted a scenario in her mind where it wasn’t a murder — it was an accident,” Winchester said.

They allegedly chose Dec. 9, 2000, as the day Winchester would take Mike on a duck hunting trip, knock him overboard and stage his death as a boating mishap.

“The plan that was discussed and come up with was that he was going to be wearing waders,” Winchester told authorities. “The belief was someone falls in the water with waders, you’re going down.“

But the morning of the trip, Mike called Winchester and backed out, according to police. When Winchester contacted Denise, she reportedly confessed she was having second thoughts.

“I was relieved, but I was like, ‘What the hell?’” Winchester told the detectives.

But days later, Denise convinced herself again to go through with the scheme, prosecutors said. Several life insurance policies — totaling $1.75 million — had been taken out on her husband, and one worth $500,000 was about to expire, according to court records.

Winchester invited Mike to go on another duck hunting trip just hours before the Williamses were set to go on their vacation.

In the early morning of Dec. 16, 2000, the two pals launched their boat out into Lake Seminole. When they reached a landing down from where they parked their vehicles, Winchester got Mike to stand up and he shoved him overboard, prosecutors allege.

“[Mike] got his jacket off and his waders off and he was in a panic, obviously,” a sobbing Winchester told law enforcement. “I was in a panic. I was driving the boat, I didn’t know what to do … and I ended up shooting him.”

Winchester then allegedly dragged his friend’s body to the shore and put him in the back of his Chevrolet Suburban. He pushed Mike’s boat back out into the water and headed back to his house, where his wife, Kathy, was still asleep.

“I got undressed. Got back in bed,” Winchester said. “And pretended that I just woke up and that I was late to meet her dad.”

He instead took off to Wal-Mart, where he bought a tarp and shovel, while Mike’s lifeless body remained in the SUV, according to police records. Winchester said he drove to a remote area at the end of Gardner Road, just miles from where Mike was raised, and buried him.

‘Sweetheart deal of the century’

Mike’s remains might still be in the wooded area if it weren’t for Winchester’s own undoing.

Without a body, law enforcement was unable to make any breaks in the case — even with persistent calls from Mike’s family to reopen the case.

Mike’s mother, Cheryl, was relentless in her search for answers, according to friends, who said she plastered street corners with posters of him for years. Experts she reached out to informed her that an alligator feeding on human remains on a cold winter day was virtually impossible.

“Cheryl knows in her heart that something happened,” Shiflett said. ”Whoever did something to Mike messed with the wrong woman.”

Denise distanced herself from Mike’s family, cutting off communication between them and her daughter, according to those close to the Williamses. But Denise’s friends said she took great care to teach her daughter about her father.

“She has also done a great job of preserving Mike’s memory with Anslee,” longtime co-worker Maas told The Post. “Anslee has grown up with a strong sense of what a great man her father was and how much her mother loved him.”

The widow, however, repeatedly refused to speak to the media about Mike over the years and, according to Winchester, talked sparingly to him about her first husband’s violent death.

“She preferred to live in la-la land, where she pretended she had nothing to do with it,” Winchester said, according to court records.

When their relationship went south, Winchester said, he became paranoid that Denise would turn him in to authorities, officials said.

In August 2016, Winchester sneaked into his estranged second wife’s car, held her at gunpoint and allegedly made her promise to take their secret about Mike to the grave.

Around an hour later, Winchester relented and allowed Denise to drive him back to his own vehicle, court records said. He drove away before she headed to the authorities and reported the armed kidnapping.

When Winchester went to trial, officials offered him a stunning deal: Spill the truth about Mike Williams and we’ll give you immunity in his case. Prosecutors also agreed not to bring up evidence fed to them by a jail informant in Winchester’s kidnapping trial.

In an hours-long interview last October, Winchester provided law enforcement with information that led them to his friend’s body.

“It was a hard decision. I think and hope it was the right one,” State Attorney Jack Campbell told the Tallahassee Democrat. “The other one was to go with the status quo we’ve had for 18 years.”

Winchester and his second wife both landed behind bars after his confession. He was sentenced to 20 years in prison for the 2016 kidnapping — and Denise was denied bond following her May arrest in her first husband’s death.

The defense declined to comment to The Post ahead of her trial, slated to start Monday. Mike’s family also declined to comment at the request of the district attorney.

Denise’s lawyer, Ethan Way, has argued that Winchester is no more than a jilted ex-husband with every reason to fabricate a story about his client.

“Brian Winchester got the sweetheart deal of the century,” Way told the Tallahassee Democrat. “He can say whatever he wants at this point. He has license to make up whatever he wants to make up.”