Foreign exchange rates are constantly changing. Indeed the FX markets are active 24 hours a day 5 days a week. There are even some middle eastern countries that continue to trade FX on the weekends.
As the market prices move so to will the banks generally move their rates around. Most banks will keep their prices the same for the entire day and it will take a movement of more than say 100 points in a day for them to change their rates. Most places are moving towards providing a real time rate so that the rates they offer you will be different all the time! This helps them to reduce their risk and also potentially provide people with better prices.