I write to let you know of a new blogpost I have written in which I introduce a new calendar correlation for the Aztec/Mexica calendar developed by Rubén Ochoa based on an original proposal by Zelia Nuttall. I think this is very likely to be the definitive correlation that solves the paradox of the apparent lack of an explicit intercalary day to correct for the leap year, while at the same time the monthly festivals doesn't seem to have moved much in relation to the tropical year and the agricultural cycle. With this correlation the question becomes moot because it assumes that the new year was fixed by observatoin of the spring equinox at the opening of the Tlacaxipehualiztli veintena, then at the end of the year number of nemontemi days was simply determined by the observation of the equinox (for example through the space between the two shrines on top of the Templo Mayor) and the number of nemontemi days thus varied between 5 and 6 - but without the need for an explicit principle of intercalation. The model matches the known correlational dates, and is even confirmed by it, since the count requires one of the years between the arrival of Cortés in Tenochtitlan on November 8th in 1519 to the fall of Tenochtitlan on august 13th in 1521 requires one of the intermediate years to have 366 days. Furthermore the count it ties the ceremonies of Panquetzaliztli (dedicated to Huitzilopochtli) to the Winter solstice when the sun was over the Huitzilopochtli temple, and the ceremonies of Etzalcualiztli (dedicated to Tlaloc) to the summer solstice during the rainy season when the sun was over the Tlaloc temple (when observed from the Quetzalcoatl temple in front of the Templo Mayor).
I will be very happy to hear what you all think.