Initially, it was thought Harry would be celebrating at a quiet dinner in the royal couple's London home with 30 close friends.
"Kate's in charge of the meal plan and William's sorting the invites," a source told Vanity Fair.
The magazine also suggests Harry's best friend, nightclub owner Guy Pelly, will throw a private party at Sloane Square hot spot Tonteria.
But apparently Kate has other ideas.
Latest reports say the duchess has teamed up with Pelly to create a nightclub inside the palace. Well, you would, wouldn't you?
"Harry has no problem with seeing his exes all under the same roof. Let's just hope they don't either," a source said. "He likes to stay friends with all his exes – so he'll be happy if she's there."
"Kate and Wills want to make it a special night as Harry moves into a new stage of his life," the source added.
The birthday celebrations are likely to start the night before Harry's party, too.
On 14 September, the prince will attend the closing ceremony of the Invictus Games, a sporting event for wounded soldiers, where the Foo Fighters, Kaiser Chiefs and Ellie Goulding will be performing at the Queen Elizabeth Olympic Park.
Sounds like September could be a busy month for the soon-to-be 30-year-old.
LOVE PRINCE HARRY? So do we...