Both the Javanese calendar and the Islamic Hijri calendar are lunar-based, meaning they rely on the cycles of the moon. This has led to questions about the correspondence between the first day of Muharram in the Islamic calendar and the first day of Suro in the Javanese calendar. The calendars share a common foundation in lunar observations, but differ in their specific calculations and traditions. Understanding this shared lunar basis is key to comprehending the relationship between these two important calendrical systems. Further explanation clarifies the nuances of their alignment and distinctions. The calendars’ reliance on the moon’s phases dictates the timing of significant religious and cultural observances within each tradition. This connection highlights the historical and astronomical influences on calendar development in the region.