Mexicolore replies: This is outlined in Book IV of the Florentine Codex, where we read (Anderson & Dibble translation):-
’They said it [7-Rain] was a good day. Merciful, pitying, and compassionate was the one born precisely upon it. He became fortunate. It was as if his fortune, his deserts, his good luck rained down upon him: as if it sprinkled on or swirled about him...’
(8-Flower, which followed, was also a good day, but then...)
’They feared the one which followed in order - Nine Crocodile.
’This, so they said, was in no way good; it was perverse, full of vice. It accorded one rags and tatters. Nothing could become a remedy or cure for the day which had followed...’
But no explanation is given as to quite WHY these days had these characteristics! Superstition played a huge part in these claims. In general 7 was a ‘lucky’ number and 9 an ‘unlucky’ one. Perhaps it has something do do with the association of these days with the period 1-Reed, which was closely linked to the wind (god). ‘And so they said it was an unfortunate time. He who was then born, whether nobleman or commoner, was of scant merit or deserts. Just as worthless as the wind was his life. It was swept away by the wind...’