Thipu1, I can't find your previous thread about MIL's big do, and I don't remember exactly what is being celebrated. Whatever it is, since it was scheduled for Father's Day, I think it would be inappropriate for there not to be some acknowledgement of the fathers there, since they did give up their own celebrations to attend. You certainly shouldn't have to hide your thoughtfulness.
Please remind us what the "big do" is about. Other than a funeral, I can't think of a family event where it would be excusable to ignore Father's Day.