This study investigates a novel calendar anomaly in the Shanghai Composite Index (SSE Composite) based on the Nayin (纳音) Five Elements system, a cyclical classification embedded in the traditional Chinese sexagenary calendar (Heavenly Stems and Earthly Branches). Each two-year period within the sixty-year cycle is assigned one of five elemental attributes: Earth(土), Metal (金), Fire (火), Wood (木), or Water (水). Using daily trading data from December 1990 to February 2025 spanning 18 complete Nayin cycles, the study finds that Earth-element periods exhibit a 100% positive return rate (mean: +123.4%), while Metal-element periods universally display late-cycle declines (4/4 periods, mean late-segment return: −30.4%). Wood element periods tend toward negative returns (mean: −11.4%). Fire-element periods contain structural bottoms, and Water-element periods mirror the directional momentum of their predecessors (3/3 accuracy). A binomial test for Earth-period returns yields p = 0.095 (one tailed), with Cohen’s d = 1.50 versus non-Earth periods. These patterns are not replicated in the S&P 500 Index under the same calendar framework, suggesting a China-specific cultural behavioral channel consistent with theories of culturally embedded market sentiment. This paper contributes to the calendar anomalies literature by documenting a previously unexplored two year cyclical pattern linked to traditional Chinese metaphysical calendrics.