Probability of a working day is defined as the fraction of workable days to all available days within a working period. So, this issue as a crucial role in farm management should be addressed accurately. This research was aimed to develop and verify a model in order to determine the probability of a work day for tillage practices in different regions by different weather and soil characteristics. One of the most restrictive factors for tillage operation is the soil moisture. In this study, a computer model was developed to estimate the soil moisture at tillage period by using meteorological data from previous years and soil characteristics data. The model was verified by a field test. In this test by inserting several sensors in 10-16 and 24-30 cm depths, soil moisture was measured and compared with simulation results of the model. Root mean squared error and modeling efficiency was found to be 2.96 and 0.822 for 10-16 cm soil depth and 4.62 and 0.711 for 24-30 cm soil depth, respectively, representing a good efficiency of the model. Also, sensitivity of moisture estimation to several model parameters was studied. The model was found to be very sensitive to n, while showed low sensitivity to α and the saturated hydraulic conductivity. Model sensitivity to primary soil moisture was medium at deeper depths and in first days of simulation and was low at shallower depths.