Legal Question in Real Estate Law in California

They have no right to refuse. You are legally entitled to enter the premises to show it. All you are required to do is give reasonable notice and enter during normal business hours. 24 hours notice is deemed reasonable. Shorter notice may be reasonable. You should also warn them that if they interfere with your sale of the property, by interfering with showings or by making the property unsightly to discourage sale, you can evict them and sue them for intentional interference with prospective economic advantage.