Most state laws require that Landlords give “proper notice” before showing up at the rental property. This rule is designed to protect a tenant’s right to privacy and “quiet enjoyment”.
The amount of notice varies from state to state, but is usually about 24 hours. In an emergency (injured person, a fire, or broken pipe), you can usually enter without giving any notice, but those instances are far and few between.
Besides being illegal, showing up without giving proper notice will irritate your tenants. They will lose respect for you, which could lead to the tenants “forgetting” to pay rent. If you want to be treated with respect, you need to treat them with respect.