Irregular rainfall often causes water shortages, resulting in reduced plant growth and grain yield. This study aimed to identify the optimal transplanting time to maximize rainfall utilization during the rice growing season. To identify the most suitable probability distributions for estimating precipitation during the rice growing season in Guilan Province, data from seven synoptic stations over a 30-year period (1991–2020) were analyzed. Daily precipitation was aggregated into 10-day cumulative intervals from Farvardin 1 to Shahrivar 30 (March 21–September 22), forming 18 decades. Box plots were generated based on statistical analysis to assess probabilistic variations in precipitation. The results showed that the Johnson SB distribution, with Kolmogorov–Smirnov (K-S = 0.11119) and Anderson–Darling (A-D = 0.49044), had the best fit for rainfall during the rice growing season in Guilan Province. The highest average rainfall with a 50% probability occurred in the second and third decades of September, measuring 56.4 mm and 61.2 mm, respectively. At the 75% probability level, rainfall during these periods is expected to be less than 100 mm and 99 mm, respectively. In the third decade of April, coincides with transplanting in very early-planted fields, rainfall with a 75% probability is expected to be less than 48 mm. Given the negative impact of rainfall in late August and September on the harvesting process, late planting is not recommended. Overall, the occurrence of adequate rainfall in spring and early summer creates favorable conditions for rice cultivation and management. Therefore, it is recommended that rice transplanting in Guilan province be conducted in early May for early-planted fields (with a 75% probability of receiving less than 43 mm of rainfall for early-planted fields) and in mid-May for fields with timely planting schedules(with a 75% probability of receiving less than 24 mm of rainfall for timely-planted fields).