Your nonprofit start page is designed to be an umbrella page with general info about your organization. The custom URL on your start page will always follow the template: www.firstgiving.com/yourorganizationame. All of your active project pages or giving day pages will link from this main umbrella page.
Your giving day or project page is specific to the campaign you are running. The custom URL of this page will allows follow the template www.firstgiving.com/yourorganizationname/givingdayname.