There is not enough information provided to answer the question.
How would I know when it was?
You would need to have listened to the story or have additional information provided in order to determine when the birthday was.
Based on the story you listened to in the lesson, I cannot answer this question as I do not have access to the story. However, you can find the answer by recalling the details from the story. The story should have provided information about the main character's aunt and whether it was her birthday or not. Listen to or review the story again to find the answer to this question.
