Father’s Day还是Fathers’ Day？

 　　Although the name of the event is usually understood as a plural¹ possessive (i.e. "day belonging to fathers"), which would under normal English punctuation² 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, 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, and it was still spelled the same way when its creator was commended in 2008 by the U.S. Congress.

1 plural   

n.复数；复数形式；adj.复数的

参考例句：

• Most plural nouns in English end in's '.英语的复数名词多以s结尾。
• Here you should use plural pronoun.这里你应该用复数代词。

2 punctuation   

n.标点符号，标点法

参考例句：

• My son's punctuation is terrible.我儿子的标点符号很糟糕。
• A piece of writing without any punctuation is difficult to understand.一篇没有任何标点符号的文章是很难懂的。