You should use the Facade design pattern when you have a complex system with many different parts, and you want to provide a simplified interface for clients to interact with the system.

The Facade pattern provides a single, high-level interface that encapsulates the complexities of the underlying system and makes it easier for clients to use.

This pattern can also help to reduce coupling between the client and the system, since the client only needs to interact with the facade, rather than directly with the various components of the system.

The Facade pattern is especially useful when you need to refactor a large and complex code-base, since it allows you to introduce a simplified interface without having to change the underlying implementation.