Q. What is it supposed to be on the show?

A. A major annual charity bash thrown by billionaire Leon Prater in his Manhattan mansion.

 
Q. Where is it supposed to be on the show?

A.  In New York City.

Q. When did we see it on the show?

A. In Episode 10 of Season One of Resurrection. 
 
Billionaire Leon Prater is throwing his 10th annual Gala, to benefit the New York Police Foundation. It is a major social occasion . There are banners draped everywhere announcing the Gala, and guests in formal dress are arriving in limousines, including many police brass and political bigwigs. 
 
The banners read: “Tenth Annual Gala: Benefiting the New York City Police Foundation” and “Celebrating Bravery. Investing in Heroes.”


Unfortunately, and unbeknownst to his guests, this giant party coincides with a time when Leon Prater has just murdered Angel Batista upstairs, and has locked Dexter Morgan in his secret vault, leaving him to die of starvation and thirst. 

This is a very long scene, which takes up almost all of the final 10th episode.

To escape from the vault, Dexter uses Batista's phone to call Harrison, and in order to get inside the party, Harrison accepts a previously-offered gig as a waiter at the Gala. So, Harrison is dressed as a server, and is handing out glasses of champagne, while Dexter uses the phone to instruct him about where to go and what to do.

So the action cuts back & forth between Harrison at the party (which was filmed inside the Duke mansion) and Dexter trapped in the vault up at the penthouse (which was filmed Inside the Guggenheim mansion, and possibly a studio set reproduction of that vault).

But along the way, Harrison is spotted and held at gunpoint by Charley. But when she learns from Dexter that Prater had a thick file about her in the vault, she cuts a deal with him that she will let Harrison live if he doesn't AirDrop those incriminating files to the police guests at the party.

Harrison eventually makes it to the penthouse and helps Dexter escape from the vault, which eventually leads to Prater winding up on Dexter's table.

( The Duke House location also plays host to the delightful scene of usually uptight Detective Claudette Wallace letting her hair down and boogieing in a red gown to the beat of Staying Alive. )

Q. What is it actually in real life?

A. An actual mansion. 

Q. Where can I find it in real life?

A. Most of the scenes for this Gala, both the exterior and interior, were shot at the James B. Duke House, a Gilded Age mansion located at 1 East 78th Street, It is literally across the street from Central Park, and just two short blocks south of The Met, on Manhattan’s Upper East Side.

 

The home was essentially a copy of the 18th century Chateau Labottiere in Bordeaux, France.

As I mentioned in my page about the brief exterior shot of Prater's  mansion, way back in Episode 4, Prater's  mansion was created by stitching together three different filming locations: a brief glimpse of the exterior of the Duke mansion in Manhattan, an impressive parking court about a mile and a half east of the Duke mansion, and most often, the interior of the Guggenheim mansion, about 16 miles to the northeast, on Long Island.

But the Gala scenes add a 4th element to that quilt: the interior of the Duke mansion.
 
Up until this scene, we had only seen the exterior of the Duke House, and even then it was only a very brief shot as Charley drove past it.

But the Gala take us inside, and the party scenes were mostly filmed inside the Grand Hall of the Duke mansion.

 
The best evidence of that is the impressive, curving staircase that is the centerpiece of the Grand Hall, but other details, such as the gold accents on the walls, complete the picture. In these Gala scenes, we are finally seeing the actual interior of the Duke mansion.
 
[ The very first bartial glimpse we got of the actual Duke interior was in the previous (9th) episode, when Angel Batista paid an unexpected visit to Prater. But this Gala scene is the first full look we've had at the interior of the actual house in Manhattan. ]

Interestingly enough, the interior of the Duke mansion and the interior of the Guggenheim mansion, which were both used to represent the interior of Leon Prater’s home, do not look alike.

But on the show, when we see the Guggenheim mansion, we are supposed to be seeing Prater’s penthouse, and its macabre vault of serial killer memorabilia, which is supposed to be on the 5th floor.  But when we see the Duke mansion, we are seeing the ground level of Prater’s mansion, with its Grand Hall event space and entrance/reception area. 
 
So it’s reasonable that the two separate levels might have differently styled interiors (although I suppose it should be noted that in real life, neither mansion has five floors, or a penthouse).
n
During the Gala, we also see Prater's parking garage again, as Dexter attempts to leave the mansion. As mentioned before, this parking court is neither the Duke mansion nor the Guggenheim mansion. It is instead located in another building on the Upper East Side.


Here is a matching StreetView of The Duke mansion: 



Here is an aerial photo of the location.  And here is a map link.