Passive solar heating of homes is a dynamic process for which solar energy is transmitted through glazing and then absorbed by the interior building components and released to the indoor air over time. This paper presents solar design days as a useful method for understanding passive solar buildings’ dynamic behavior for the purpose of increasing energy performance and thermal comfort through interactive design at the conceptual design stage. Rather than relying on rules of thumb or assessing whole-year performance from building energy simulations to optimize passive solar measures, solar design days can be used to compare different design options by diagnosing potential problems such as excessive heat loss, peak loads, and overheating. Solar design days consist of representative cold sunny, cold cloudy, warm sunny, and mild sunny days. This paper provides a background on recent advances in passive solar design, a methodology for selecting and applying solar design days, a modeling approach for passive solar houses in EnergyPlus, and finally, an example.