In "To Kill a Mockingbird" why did some African Americans forget their birthday or were not sure it was correct?Extra Credit: if I know more about...

Many black people during this time period could not read or write; because of this, they didn't use traditional calendars like we have today; they wouldn't be able to read it.  They kept time in different ways, through the position of the sun, length of the days, and seasonal signs related to the planting of crops.  Because of this, Calpurnia didn't really know the exact day of her birthday.  She states in the book,

"We started rememberin' one time, trying to figure out how old I was...I just have [my birthday] on Christmas, it's easier to remember that way-I don't have a real birthday."

Of course she had a real birthday, but didn't know which day it was on, or even which year.  Keeping track of such things was not as significant for large, poor families back then.  Who could afford to celebrate a birthday, even if they did know how to read the date when one was born?  So, Calpurnia figures that she's "older than Mr. Finch, even...not sure how much though," and that is good enough for her.