Late spring frost is one of the most significant climatic hazards affecting the sustainability of apple production in the orchards of Semirom County, Iran. This study aimed to statistically analyze the temporal occurrence pattern of late spring frost events and identify critical temperature thresholds during the sensitive phenological period from 5 April to 27 April over a 20-year period (2002–2021) using observations from the Semirom Synoptic Station. Daily minimum air temperature data were quality-controlled and classified into three frost severity categories (mild, moderate, and severe). In addition, the annual dates of the first occurrence of the −1 °C and −2 °C temperature thresholds were identified. During the study period, a total of 27 mild frost days (5.8% of the observation window), 23 moderate frost days (4.9%), and 3 severe frost days (0.6%) were recorded. Long-term trend analysis using the Mann–Kendall test and Sen’s slope estimator revealed no statistically significant trends (p > 0.05) in frost occurrence across any severity category. During the recent period (2014–2021), the −1 °C threshold was exceeded in only 3 of 8 years, with a mean occurrence date of 11 April, whereas the −2 °C threshold was not reached in any year. In contrast, during the earlier period (2002–2013), both thresholds were exceeded in all 12 years. Although these findings suggest a substantial reduction in the frequency and intensity of late spring frost events, they should be interpreted cautiously because of the single-station dataset and the relatively short observation period compared with the standard 30-year climatological baseline. The results provide a quantitative basis for annual frost risk monitoring and support the development of adaptive frost protection strategies for apple orchards in the Semirom region.