The ritual calendar(s) are quite detailed, with minor differences between cultures. The Maya were no special case in the following list, because the ritual calendar was more-or-less driven by the imperialism of the Aztec empire.
Simply, the 'corners of the year' (solstices and equinoxes) were the major ritual times because the sacrifices governed the prosperous / abundant transition of the seasons.
Omens were another large governor of sacrifices:
- volcano eruptions,
- eclipses, etc.
Not all sacrifices were human, and other events called for blood (cutting) or animal sacrifice. Such non-human-death sacrifices were often evoked by personal omen-istic experiences like
- auspicious timing of personally-experienced events,
- witnessing of clouds,
- 'instructions' received during psychotropic medicine,
- anthropomorphic plants, etc.
Because of the complex inter-linking of planetary calendar phases (sun, moon, venus, mars, saturn, mercury), certain calendrical intersections were times for sacrifices. Places where the count reset itself (such as the 2012 long-count incrementation) were also times for awareness.
Since many of these intersections are dictated by solar cycles, the seasons sacrifices would have had special meaning at these epagomenal times.