If you don't know and can't learn them in time, you have to turn down the work, unfortunately. Certainly for concerts with an orchestra, until you get really established - and sometimes even then - the phone call will be 'we want to play this concerto in a concert this season, can you play it?'
Unless you're playing a solo recital in your own right, you will never have complete freedom over your concert repertoire because you have to choose it in conjunction with other musicians.