An apostrophe before the ‘s’ usually indicates the singular form of the noun; an apostrophe after the ‘s’ usually indicates the plural form of the noun.
All the cards in the shops refer to Father’s Day or Mother’s Day, as every child has one (singular) father / mother. Therefore, the apostrophe is placed before the ‘s’.
The card companies want to personalise the event. As such, the apostrophe before the ‘s’ reflects the special one.
As all (plural) fathers / mothers around the world are being celebrated on these special days, the apostrophe should be placed after the ‘s’.
To avoid any doubt about the correct placement of the apostrophe, allow me to make a suggestion with regard to a possible wording on the card.
Apostrophe after the ‘s’:
This wording satisfies both the individual nature of the event and the generality of it all.