You have a new product that is ready to launch, a new service, a new ad campaign, or a new website… at what point do you promote it internally?

You serve food, when you are sure that it is ready, looks yummy and palatable, irrespective of it being served to the family (internally) or to the guests at a party (externally, in the market).   

Same with the product launch.

At the same time/ day you launch the product/ service/campaign...

Usually, it is very important to keep these things confidential until the launching date, mainly to prevent your competitors from knowing what you are up to.

I think A new add campaign  is more effective for new product ,

that show the creativity of the company and product attributes.

You have to start the campaign without giving opportunities for your competitors to  influence in the market with ideas that may torpido your efforts but at the same time it should register in the minds of prospective customers and the market as a whole.  The timing depends upon case to case. A well matured balance to keep in the launching process ensure your success to score more.