Salvia officinalis L. (common sage) is a widely used medicinal and culinary plant, but seasonal and diurnal fluctuations in its essential oil composition pose challenges for safe year-round utilization. Compounds such as alpha- and beta-thujone and camphor may reach toxicologically critical levels, whereas 1,8-cineole is generally desirable. To determine safe harvest windows, we quantified monthly (April-January) and diurnal (twelve 2-h intervals) variations in essential oil composition under semi-arid conditions in Mardin, T & uuml;rkiye. Essential oils were isolated by hydrodistillation and analyzed using gas chromatography-mass spectrometry. Results showed that 1,8-cineole peaked during spring and late summer, while thujones and camphor were more abundant in colder months. Diurnal trials indicated higher cineole in daytime harvests and elevated thujone/camphor at night. Factorial ANOVA confirmed significant season & times; time interactions, and PCA clearly separated cineole-rich summer/daytime samples from thujone/ camphor-rich winter/nighttime samples. Harvesting during spring-summer mornings or evenings is therefore recommended to optimize both safety and quality of sage oils, whereas winter harvests may require additional consideration due to relatively higher thujone and camphor levels. These findings provide practical guidelines for safe utilization of S. officinalis essential oils in semi-arid regions.