I've heard that sometimes people had the name of the day they were born, for example 15 Water would have been born in the 15th day of the month called Water
But then why did some people have names based on the calendar and others didn't?
Today in Mexico there are many people with aztec names like Xochitl, Tonatihu, Atzin... but no one uses those calendar names
Hello, the name would be the day-year and then the name-name. Sometimes people were known not the name given by the parents, but the name they acquire due to the circumstances of their lives. Or worse, by the name the Spaniards who met them could pronounce. So Motecuzomatzin Xocoyotzin was called Montezuma or Mutezuma.
There have been a lot of studies about the names, but remember that the sources for the most part, are the very misunderstood Spaniards who wrote the books afterwards.
What we know is that the day the child was born (unless it was a "bad" day) was the name given, and then after 4, 6 or 7 years they were given a family name, which might be changed if their lives did not reflect that name. The name was very important, and when the child was born, they consulted a namer or priest who would tell them the future that was destined to the child. (As quoted in Duran, below). But before that, the doula would also name the child according to the day the child first saw light. So the child had 2 names before they were given an actual name. For the first years of their lives they were called cotton or something cute like that.
So for example the priest Quetzalcoatl who, the one who was to return, was named Ce Acatl Topiltzin Quetzalcoatl. Or One-Reed Topiltzin Quetzalcoatl. He was supposed to return in a year Ce Acatl, same as he left.
Netzahuatcóyotl was named Acolmiztli Nezahualcoyotl Cemazatl (Or Acolmiztli Nezahuatcoyotl 1-deer). His name given by his parents was Acolmiztli (strong feline), but he had a hard life and was renamed the fasting coyote (Nezahualcoyotl).
Cuauhtémoc (Cuāuhtémōctzīn) was called Guatemuz by the Spaniards.
On regards of current Aztec names, it depends on the parents or the personal choice. For example, Naui Ollin's name is a calendar name. Xochitl (flower), Tonaitiuh (Sun), and Atzin (Water) are common pretty names too, but refer to things or places.
Duran's interpretation or description of the naming of the Aztec children:
servían para mirar las venturas buenas o malas de los que nacían, y así que,
en naciendo que nacía el niño o niña, iba lue go el padr re, o parientes del
nacido a los astrólogos, hechiceros y sortilegios que los había sin número, y
rogábanles les declarasen la ventura en que su hijo, hija, habían nacido,
siempre llevando por delante la ofrenda de com ida bebida
suertes y calendario, y vista la letra del día, pro onosti o caban ech aban
suertes y decíanles la ventura, buena o mala, según había oído la suerte te.
Porque la ciencia de su astrología y quiromancia no se extendía a más de un
papel pintado de cuantos ídolos había, y adoraban donde tenían cada ídolo en
su casa, es a saber: al dios de las mieses en su c asa, al de la riqueza, en
la otra, y al de la penitencia, en la otra, y al de la lujurio, en la otra, y
al de la borrachera, en la otra, al de la guerra, en la otra, y al dios del
culto de los dioses, en otra.
calendario; sobre este papel echaban suertes 'e, conforme caía, pronosticaban.
Y si caía la suerte sobre el dios de la vida, decían que era de larga vida; si
caía sobre la muerte, decían que había de pedir poco, así de los demás, que
por quitar prolijidad, no pongo en cada uno en particular Baste saber que, si
había de ser rico, o pobre. o valiente, o animoso, o cobarde, religioso, o
casado, o ladrón, o borracho, o casto o lujurioso, allí en aquella pintura y
suertes lo hallaban avisaban a los padres y palientes, hacjéndoies salvas
primero, y pláticas largas i retoricas salían después con dos docenas de
mentiras y fábulas, afirmando cosas que, aun al diablo que les persuadía
aquello, le es oculto, pues sólo a Dios son las cosas futuras presentes.
(in D. Durán, https://repositorio.tec.mx/bitstream/handle/11285/573953/DocsTec_7313.pdf?sequence=1&isAllowed=y)