Plan ahead for the fact that this may happen. When you first contact a venue and set a date, ask your client if the venue has one or two possible alternative dates. Don't be afraid to ask about changing a date. Experienced clients understand routing problems and will accommodate your request if they can. And besides, you never know, sometimes changing a date at your request also solves a routing problem that the client is having.
Clients deal with routing problems on their end as well, and they are often juggling dates.
On occasion, a client may even call you to ask if you can change a date. Do your best to help your client out if you can. Cooperation will help you establish a positive reputation in the business and a solid working relationship with the client. When you're considering swapping dates, keep in mind that gigs with signed contracts can't be changed. Some venues are locked into dates that can't be changed.