YES you have a right to show your property to prosepective tenants.
You should give the tenants 24 hours before you show it or have an open house for a couple of hours on saturday and sunday....
Here is the tricky part.....You are entitled by law not to lose one days rent.
So if you wait till they move out, the right tenant might not be able to sign a lease for a month.
So you have some options.......
Is the tenant breaking a lease? then the tenant is responsible for all rent lost, so let them decide if they want to pay you next months rent, and you only show it after they vacate.
If not, the lease is expired in 30 days....you could give the tenant ALL of their security back if you can show the house.
It is a business you have a right to re rent but YOU also take a chance you wont find a great tenant to replace them the day after their lease expires.
Remember the tenant feels that you are really imposing on them by you doing this, And if the tenant is happy, they will convey that to the prospective tenants, so paying them back ALL of thier deposit might be a very wise business thing to do.