Why did a temporary custom of observing an extra festival day evolve into a binding religious practice for Jews living outside Israel? The elevation of Yom Tov Sheni (Second Festival Day) from minhag to takanah status represents a fascinating intersection of practical necessity and theological meaning in Diaspora Judaism. Through systematic analysis of talmudic literature, medieval commentaries, and responsa, three primary halakhic justifications emerge: preparing for potential restoration of the Sanhedrin and lunar calendar determination, establishing Israel's spiritual centrality, and protecting Diaspora Jewish observance. The research demonstrates how Yom Tov Sheni functions as both symbolic and practical infrastructure, maintaining the theological distinction between Eretz Yisrael and galut while reinforcing Zion-centered Jewish consciousness. While acknowledging legitimate concerns regarding observance difficulties, the analysis reveals that eliminating the second day would undermine broader Jewish calendar observance and weaken the halakhic foundation supporting Diaspora Jewish life. The findings support retaining Yom Tov Sheni as minhag rather than obligatory chok, thereby accommodating varied community needs while preserving this essential element of traditional practice.