Their date of birth ?
Yes, their date of birth. Therefore, the nahual of every person is determined by the day and date of birth. Therefore, that’s the charge that the person has. When a baby is born ... The nahual [of every person] is an energy, a knowledge.
As Odilia mentioned, when a person is born, the day he or she is born on is commonly referred to as her or his nahual. My birthday, 11 February 1988, is 11 Ajpu - this, then, is my nahual. A person born 31 May 1992 would be a 9 B’atz’, and so on. Since the nahuales are regarded to have certain characteristics, the day one is born will influence one’s own abilities and characteristics - much like similar thoughts within Western astrology.
A person born on Tz’ikin, for instance - that’s a day of wealth and riches - will be very rich or very poor, generally. But it’s an either/or. Usually their economic situation will play a big part in their life - either positively or negatively. And a lot of that depends upon ourselves - depends on the kind of relationships we develop with the divine, the kind of relationship that we develop with our own nahual, for instance.
The nahual of a person determines several characteristics, abilities and possibilities of that person - both advantages and disadvantages - but it is not a deterministic thought, only a guide. One decides for oneself whether one takes the opportunities one is given.
There is no determinism or indeterminism, but self-determinism. A man is the cause and effect of his own destiny. There is nothing that says I’m going to be an engineer, or I’m going to be a drunk, no. It’s your “chip,” your mind that is going to decide - with your capacities, your potential, your energy, your science, all of it. That’s what’s going to construct your life. And to make it, there will always be conditions. [Only] those who have a sea can fish in it, [but it is their choice to do so].
All of us make our own lives - to some extent - but we’re all also born with some things. You were born in Norway. You were born a man. You were born in the 20th century. And so that means that you start off with some “stuff”. And after that you make all sorts of decisions in your life. Everybody around you make other sorts of decisions which affect your life. So some things you’re born with, and some things you create, and other people create for you.
And it’s the same with the nahual. You’re born with some things that you’re not going to change. Other things you are going to change according to what you do, and how you live, and how those around you live, and what you’re government decides to do as far as educational opportunities are concerned. So as I said, there’s part of us that is an inheritance, part of us that depends on the day we’re born, part of it depends on what we do with ourselves and what others do with us. That’s the way a nahual is.
Certainly, a nahual brings with it risks and potentialities, it brings strengths and weaknesses. We then decide what to do with those. That affects how we live our lives, how we decide to live our lives, and it affects what’s going to happen to us.
Most ajq’ijab, including those I have talked to, use books with tables to determine the day’s nahual by “translating” from the date in the Gregorian calendar. But several of them told me that there are ajq’ijab who do not use tables - instead, they “sense” the person’s nahual when they meet them.
-  An example of such a table can be found in Barrios, The Book of Destiny: 298-338.This is the table I have used for the dates above. Carlos Barrios is an ajq’ij who hasspecialised in performing ceremonies for tourists and writing books for a Westernaudience.