Archaeoastronomical studies have demonstrated that the important civic and ceremonial buildings in Mesoamerica were largely oriented to sunrises or sunsets on specific dates, but the origin and spread of orientation practices were not clear. Using aerial laser scanning (lidar) data, we analyzed orientations of a large number of ceremonial complexes in the area along the southern Gulf Coast, including many recently identified Formative sites dating to 1100 BCE to 250 CE. The distribution pattern of dates marked by solar alignments indicates their subsistence-related ritual significance. The orientations of complexes built between 1100 and 750 BCE, in particular, represent the earliest evidence of the use of the 260-day calendar, centuries earlier than its previously known use in textual records.