Prasat Hin Phanom Rung, located in Buriram Province of Thailand, is an ancient temple that had been built between the 10th and 13th century. The temple, which is off east-west orientation by 5.5° towards north, has unveiled the astonishing phenomena exhibiting both astronomical and architectural intellect of the ancient builders. The phenomena involve perfect quarterly-alignments of the sun through all the fifteen doorways of the temple. The phenomenal orientation of this ancient architecture has been elucidated by several scholars—including historians, archaeologists, and astronomers—that it might be related to solar or lunar events only. However, our studies have otherwise found a clue to this mystery that it may be based on how the ancient intelligence used stars in the zodiacal constellations to regulate agricultural calendars. In this study, we find that Phanom Rung was oriented with respect to Spica such that on the day Spica set on the west-side doorway at dawn, the sun was entering Mesha Rashi (Aries). This day has a direct connection to a New Year’s Day of Saka calendar (Śaka Era), presently called Thaloeng Sok Day. Furthermore, we have found the relationship between Spica and the full moon of Caitra from which the intercalary month-year (Adhikamas) was detected.