Short answer:
as early as you possibly can.
Long answer:
there is a very limited number of top notch experienced conference interpreters on the high end market and they are usually busy for up to several months in advance.
Most professionals we work with are freelancers, so they will accept the first firmly confirmed assignment.
An assignment is confirmed when we sign a contract with our client.
In our past experience, it is virtually impossible to find an experienced interpreter at a short notice, so if you want to book the best conference simultaneous or consecutive interpreters, you need to start early.
Also, to insure quality we may need to fly interpreters in for your conference depending on particular circumstances, for example, language combination.
