Yucatan Before and After the Conquest: XXXIX. The Author's Explanation as to Various Things in the Calendar. His Purpose in Giving These Things Notice
Together with the characters of the Indians shown above in our chapter 100 (Sec. xxxiv), they gave names to the days of their months, and from all the months together they made up a kind of calendar, by which they regulated their festivals, their counting and contracts as business, as we do ours; save that the first day of their calendar was not the first day of their year, but came much later; this being the result of the difficulty with which they counted the days of the months all together, as will be seen in the Calendar itself we shall give herein later. The reason is that although the signs and names of the days of their months are twenty, they were used to count them from 1 to 13; after the 13 they return to 1 again, thus dividing the days of the year into twenty-seven thirteens or triadecads, plus nine days, without the supplementary ones. With these periodical returns and the complicated count, it is a marvel to see the freedom with which they know how to count and understand things. It is notable that the dominical always falls on the first day of their year, without fail or error, no other of the twenty ever taking that position. They also used this way of counting to bring out by the aid of these characters a certain other count they had for their ages; also other matters which, although they were important for them, do not concern us much here. We shall therefore be content with saying that the character or letter with which they began their count of the days or Calendar is called Hun Imix (One Imix), which is this:
Among these people the first day of the year always fell upon our 16th of July, and was the first of their month Popp. Nor is it to be wondered at that this people, however simple as we have found them in many ways, also had ability in these matters and ideas such as other nations; for in the gloss on Ezekiel we find that according to the Romans January began the year, according to the Hebrews April, according to the Greeks March, and according to the Orientals October. But, although they began their year in July, I put their calendar here in the order of ours, and parallel, so that our letters and theirs will come noted, our months and theirs, together with their above-mentioned count of the thirteens, placed in the order of their progression. *
68:* In the pages following, the manuscript sets out in full each of the successive 365 days, 
and which has no fixed day on which it must fall. For each modifies its own count, and with all this the dominical letter as they have it never fails to fall on the first day of the following year.