Heatwaves pose significant threats to human health, infrastructure, and natural environments, with documented increases in their occurrence globally. However, previous studies often focused on summer heatwaves during fixed periods, potentially overlooking inter-decadal variability and the contribution of shoulder seasons (e.g. spring and autumn). This study provides an updated analysis of global heatwave characteristics using reanalysis data up to 2023. We examined trends in heatwave frequency (HWF), intensity, and duration at multiple spatial/temporal scales, investigating how trend patterns and magnitudes vary across different periods. Results reveal substantial spatial heterogeneity in total heatwave days, with maxima exceeding 24 d per year in the Brazilian Highlands, the west coast of North America, parts of southwest Africa, and India, while cumulative temperature anomalies are highest at high latitudes. Notably, boreal summer is not consistently the season of most extreme heatwaves; over 65% of land areas experienced maximum HWF and severity in non-summer seasons. Long-term trends reveal an accelerating increase in HWF and severity globally, with exceptions in parts of India and central North America. Furthermore, varying-period analyses highlight substantial inter-decadal climate variability. These findings emphasize the need for seasonally adaptive risk management strategies and updated climatological baselines to address the evolving threats of heat extremes in a warming world.