Why do you think it was important for slave owners to keep slaves ignorant about their birthdays and parentage? douglass opens his story by telling us that he is troubled by not knowing when he was born. why is this fact so important to him?

The slave owners kept them from having an established identity. If they saw themselves as individuals with a purpose and sense of self, they would then have the desire to question authority. They kept parents and children seperate due to human instinct to develop kinship ties; the slaves would form a support system and thus establish a group identity, leading to potential uprising and questioning their positions. Education is freedom, as is knowledge. Why give such a gift to those in servitude? (Rhetorical question...) 

The slave owners didn't want anyone to really know who they we're  they also didn't want them to rebel,