The belief in the influence of celestial bodies on the lives of earthly people is one of the enduring elements in the history of Iranian and Islamic thought. Among these bodies, the moon has played the greatest role in determining good and bad days due to its proximity to the Earth. Islamic astronomical texts, especially in the astronomical rulings, clearly state that the geometric ratios between the moon and the sun - such as the Conjonction period (mohaq), Pleine lune (badr), and the quarter lune periods (tarbi'ah) - were considered inauspicious and ill-considered times. These situations were described with terms such as “hostility” and “semi-hostility” and were considered to impact individual and social life, from marriage to court politics. The present study seeks to address the question of whether this ill-considered day is simply a product of Iranian astronomical knowledge during the Islamic period or has its roots in pre-Islamic Iranian beliefs and chronology. A study of the Mazdasenaite chronology shows that the classification of the days of the month by repeating the name of Ahura Mazda on the first, eighth, fifteenth, and twenty-third days is not unrelated to the lunar manifestations and was in some way an attempt to eliminate the inauspiciousness of those days. Abu Rayhan Biruni’s reports in his works al-Baqiyya also testify that pre-Islamic Iranians considered conjunctions and full moons to be inauspicious. Therefore, the present study, using a historical-analytical method, shows that the negative perception of lunar manifestations in Islamic astronomy is closely linked to the ritual and calendar heritage of pre-Islamic Iran and reflects a significant cultural continuity.