This paper presents a rationale for and description of the professional development system designed to help teachers understand and use the Birthday Party (BP) Mathematics Assessment, a standardized assessment with child-friendly birthday party themed tasks, and ultimately to leverage their learning from the BP to conduct their own meaningful formative assessments to meet their specific instructional needs. The professional development system includes both in-person workshops and a website to help teachers learn about the BP, develop interviewing skills necessary for formative assessment, and gain a deeper understanding of the process of formative assessment. The workshops and website are based upon a research-based set of principles for professional development and rely on videos that portray young children's responses to formative assessment questions, researcher descriptions and illustrations of interviewing skills, and information about critical mathematics content for preschool teachers. Finally, we present findings from a pilot study of the website and suggest areas for future development and research. (author abstract)