I know this has been discussed at length on another thread and the reason given for not using the apostrophe was to be consistent with the organisation that started the celebration of the contribution of grandparents.
 
But Grandparents' Day was introduced in the UK in 1990 by Age Concern and they use the grammatically correct apostrophe - see page 2:
 
http://www.ageconcernsirgar.co.uk/assets/downloads/Age-concern-sir-gar-july-10-n ewsletter.pdf
 
It would be nice if we could have some flexibility in this to cater for places outside the US.