Pistachio (Pistacia vera L.) is a crop with a high economic importance that is expanding in the Mediterranean basin. Understanding how pistachio growth stages vary with environmental conditions is crucial because decisions about irrigation, fertilization, and pest control must align with tree physiological needs at each stage. However, a description of the phenological stages of pistachio is lacking in the areas covered by this study. In this study, the growth of the fruit and kernel were measured in three pistachio cultivars: ‘Kerman’, ‘Golden Hills’ and ‘Lost Hills’ in six plots located in central Spain to establish predictive models based on heat accumulation. The growing degree days (GDD) necessary to reach 90% full fruit area were similar for the three cultivars, around 450 GDD. In contrast, the thermal units required to reach 90% of full kernel area and length differed among cultivars: ‘Kerman’ had the highest needs, while ‘Lost Hills’ had the lowest. Fruit dry weight started at 200 GDD and continued until 2000 GDD, whereas kernel area started its expansion at 900 and continued until 2000 GDD. These findings suggest that deficit irrigation could be applied between 450 and 900 GDD, without compromising pistachio yield. A model previously proposed in California underestimated both fruit volume and kernel length when applied to our calibration dataset. Conversely, the model calibrated in the current work performed better and can be considered satisfactory for assessing kernel length and fruit volume as a function of cumulative GDD. These results provide essential insights into the phenological development of pistachio growing in cold-winter regions, aiding farmers to take informed decisions for an efficient orchard management.