We present the user-centered design and testing process of a mobile, location-aware event planner. Using questionnaires, interviews, and discussions with potential users, we investigated the ways individuals plan social events, such as business meetings, dinners and gatherings, and perform the attendant communication tasks. We catalogued the contextually dependent ways in which people plan their meetings and informal social events and devised a wide range of conceptual sketches to address our potential users professed needs. We developed a preliminary PDA-based prototype based on clear conceptual entities (people, places, locations and events) and designed to support short interaction sequences and large maps. Its evaluation with a group of nine participants provided qualitative feedback and prompted an interface redesign. This development process taught us a number of interesting lessons applicable to this category of location-aware applications, which we report at the end of the article. The set of design recommendations can assist designers in the development of location aware systems.