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How did the 365-day solar calendar combine with the ritual calendar?

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Map of Mesoamerica showing the geographic distribution of various Indigenous peoples, with regions labeled for groups such as the Maya, Nahua, Zapotec, Mixtec, Otomi, Mazahua, Huastec, Tarasco, and others across present-day Mexico and parts of Central America. Boundaries indicate the approximate areas inhabited by each group. — Mesoamerican Indian Distribution of Meso-American Indians.

In Mesoamerican civilizations, the 365-day solar calendar and the 260-day ritual calendar (often called the Tzolkin among the Maya or the tonalpohualli among the Aztecs) functioned concurrently to track time and determine the fate of days.1234

These two cycles meshed together to form a longer, repeating period of 18,980 days, known as the Calendar Round. Because 18,980 is the least common multiple of 260 and 365, this cycle lasts for 52 years. Within this system, any specific combination of a ritual day and a solar date would only recur once every 52 years, providing a unique designation for each day within that period.124

While the 260-day ritual calendar was primarily used for divination and determining the auspicious or fateful nature of specific days, the 365-day solar calendar—divided into 18 months of 20 days plus five unlucky "nameless" days—helped organize the year and fix the timing of festivals and agricultural activities.1234