D. They had days designated for religious observances.
Explanation:
<em>The one thing Spanish and Aztec calendars had in common was the designated dates for religious observances</em>. The Aztec calendar was composed by two calendars, one that specified which days were good for harvest conditions and one for their religious culture, the Aztecs worshipped several gods, and their calendar provided dates for each god's ceremonies. The Spanish calendar also known as Julian calendar and afterwards as Gregorian calendar had designated dates for feast days of Catholic saints, this calendar is the most widely used nowadays.
