Technical Abstract: Accurate information on the suitable days available for field operations is important in the design, development, and selection of efficient machinery systems for crop production. The number of days suitable for fieldwork is closely related to weather and soil moisture, so values vary widely with climate and soil characteristics. A model is available that predicts suitable day information from long-term weather records and soil characteristics for essentially any location. This model forms a component of a whole farm model where it is used in the simulation of the timeliness, productivity, and costs of machinery systems in crop, dairy, and beef production. Optional output provides annual, long-term average, and 80% and 90% probability values for the days suitable for fieldwork each month over many years of weather. The model was verified to predict suitable day information similar to field observations for recent years in northwestern Indiana and similar to historical data for a few other locations across the Midwest. The number of suitable days predicted each month was moderately sensitive to the characteristics specified for the soil and highly sensitive to the tractability coefficients used to determine a suitable day. Recommended tractability coefficients were developed for spring and fall operations on various fine and coarse textured soils. The model provides a useful research and teaching tool for studying the influence of weather on the performance of field machinery operations and their effects and interactions on other parts of the farm.