The other play off semi will be between Swansea and Nottingham Forest.
For Cardiff and Forest it's a consecutive play off campaign. Cardiff made the final last year losing to Blackpool who had beaten Forest over two legs in the semis.
Cardiff travel to Reading for the first leg on Friday May 13th, ko 7:45pm at the Madejski.
The second leg in at Cardiff City Stadium on Tuesday May 17th, ko 7:45pm. Both legs are live on Sky TV.