The Xiuhpohualli is the ancient Aztec calendar that tracks the solar year. It consists of 18 months, each with 20 days, plus an additional 5 "empty" days, making a total of 365 days. Each month has its own unique festivals and rituals, reflecting the agricultural and religious practices of the Aztec civilization. This calendar is essential for understanding Aztec culture, as it helps to organize time around important events like planting and harvesting. The Xiuhpohualli is often paired with the Tonalpohualli, a 260-day ritual calendar, creating a complex system that guided daily life and spiritual practices in Mesoamerica.