From Wiki:
Although the name of the event is usually understood as a plural possessive (i.e. "day belonging to fathers"), which would under normal English punctuation guidelines be spelled "Fathers' Day," the most common spelling is "Father's Day," as if it were a singular possessive (i.e. "day belonging to Father"). In the United States, Dodd used the "Fathers' Day" spelling on her original petition for the holiday,[4] but the spelling "Father's Day" was already used in 1913 when a bill was introduced to the U.S. Congress as the first attempt to establish the holiday,[13] and it was still spelled the same way when its creator was commended in 2008 by the U.S. Congress.[21]