LOS ANGELES – It’s the day before Charlie Sheen’s 39th birthday, and Denise Richards has a party to plan for her husband. The nurse she hired to help take care of their 6-month-old daughter gave her the willies and had to be let go, and her housekeeper/nanny is only part-time, so tonight Richards will be on full-time baby duty.

The heat is unbearable. And in just a few hours, Zone diet dinners will be delivered to Denise and Charlie’s Agoura Hills mansion, and she had better be home to receive them.

You’d think all this insanity would leave a person flustered, but not Richards, who’s cool, calm and ready to talk up her TV movie, I Do (But I Don’t). It aired yesterday on Lifetime (9 ET/PT).

The film, based on a book Richards, 33, never read, casts her as a wedding planner assigned to a true Bridezilla and her philandering party-boy fiancé.

Hmm. Could there possibly be some parallel to Richards and Sheen’s own courtship?

Not according to Richards, who swears that by the time she met Sheen, he was a completely reformed, sober, true-blue gentleman.

“A lot of people ask me if his past bothers me, but I can’t even comprehend it,” she says. “The other day I said to him, ‘I can’t even picture you having a glass of wine.’ ”

Naturally, Sheen’s model behavior helped their wedding in June 2002 go off without a hitch, as did having the famed Mindy Weiss as their personal wedding planner.

“I have so much more respect for her job (now) having played one,” Richards says. “It’s a big job putting a wedding together. Not a thing went wrong with ours.”

The addition of baby Sam last March has only strengthened Richards’ bond to Sheen, who willingly changed diapers, provided midnight feedings and made regular trips this summer to the Montreal set of I Do (But I Don’t) to be with his girls.

“Sam’s really changed our lives for the better,” Richards says. “I’ll catch (Charlie) in her room, rocking her, making faces and singing silly things to her.

“He and I love each other (even) more than we did.”

So confident is Sheen in his marriage that he encouraged his wife to strip down for the December cover of Playboy. The shoot took place last month in the Bahamas and finds Richards wearing … sand, and that’s about it.

“He was very supportive,” she says. Sheen helped her choose the final photos. “Charlie’s been a Playboy fan for years.”

But the sexy shoot meant Richards had to drop her pregnancy weight pronto.

In addition to her Zone diet, Richards hired a trainer for daily cardio workouts, which included running in the sand, jumping rope and lunges.

With her C-section scar having healed well, all she had to worry about were sagging breasts. Although she tried breast-feeding, she abandoned it after the first few months.

“I felt like I was still pregnant,” she says. “I still had to watch what I ate, and I couldn’t have a cocktail, which I was dying for after being pregnant nine months. I wanted to feel like myself. You’re trapped when you always have this kid attached to your boob.”

But Sam’s got to eat something, which means Mom must now learn how to cook: “It’s not like I can expect her to eat our Zone meals.”

From Elites TV: DENISE RICHARDS selected an unusual entourage to accompany her to the Bahamas for her PLAYBOY photo shoot – her mother, father and six-month-old daughter SAM.

WILD THINGS star Richards flew to the Caribbean islands recently to pose for the sexy snapshots and took her family with her, although they relaxed by the pool while she was photographed.

And Richards, wife of actor CHARLIE SHEEN, admits that Playboy magazine has held a memorable place in her family’s history for a number of years.

She explains, ‘My dad subscribed to Playboy when I was little.

My sister and I found a magazine and we were really upset. We thought my dad was gonna divorce my mom.

‘So he sat us down and had a talk and explained that he buys the magazine for the articles. So I thought it was very interesting how this came full circle: now the daughter’s on the cover!’