I believe it's the responsibility of the venue to provide the interpreters. However, it's probably up to you to make a formal request for one. It's not automatic.
The agency I used to work for was very proactive about going out and getting contracts for interpreting services with all the hospitals, colleges, theaters, auditoriums, museums, etc. They offered discounts and trial (free one-time) services, and educated the businesses and services about interpreting and the ADA.
