When should I actually tell the MIL that sons birthday

[babyicebean]

is NOT on the 31st of February?

She has a great habit of mixing the grandchildrens birthdays up with random dates so she decidied to write them down.

She got the dates in April and she still hasn't realised.

Am I mean to let her stew?

mmm i'm not sure
How old is she ...is she a bit errm forgetful?
I have 3 relatives that think that ds is a year younger than he is and have sent him the wrong age card every year for about 4 years.
I haven't told them...but then i'm thinking when he gets to 18 they will be upset if they get it wrong.
I would probably tell the mil about the wrong date
errm...i think

Well I wouldnt bother because she is soon going to realise there isnt a 31st of February and then she will probably ask you for the right date

She is not forgetful just totally batty.

Embroider her a '28 days has September...' sampler for her next gift

No need to tell her, unless his birthday happens before the end of Feb as you wouldn't want her to miss one.

Sorry I should have said his birthday is the 16th of feb.

AH, then ignore for many many months (pretend you don't know what crazy things she has written in her book) then nearer the time start a conversation about how 'DS's birthday is 2 weeks on Thursday'.