Hello!
The word "defined" means to be detailed and/or thoroughly explained.
The words "ill defined" mean quite the opposite. Ill defined means that what is being described lacks information and is unclear.
When the author says "These days are ill defined in my memory," it means that the author has trouble clearly remembering the days that he/she is referring to. He/she cannot vividly remember the picture that he/she is thinking about.
I hope this helps you! Feel free to comment or PM me if you have any additional questions. Have a great day!
