Double Dad's Day

Double Dad's Day is an episode whose title card is simply grammatically incorrect.

Double Dads' Day would make more sense, because "Double Dad" is not a single person, "Double Dads" are the 2 fathers of Luc named Bayani and Paul.

When a plural word (or other word ending in S) is conjugated in the possessive form, an apostrophe is put at the end of it.