In normal conversation 的 isn't used for relatives or family （我叔叔，我媽媽，我爸爸）

There is only one 的 character on the android ap and I used it as follows; 叔叔的生日.

I placed 的 incorrectly at: 今天 是 我的 叔叔 生日. In the above, could someone explain what it means or why it is incorrect.