SweetSunshine:

My mum has been dead for twelve years and my Dad told me the other day that my sister had called him thinking Mummy's birthday was on the 23rd of this month ... when it was on the 23rd of last month ... and she was going to go and visit our grandmother for it (mum's mum) ...!

My sister is quite amusing when she wants to be and she was like thanks dad for saving me the embarrassment ...

it's normal IMO