There is no consensus on whether to use an apostrophe in Fathers Day.
Take any group of people and ask them this question. Does Fathers Day need an apostrophe, and — if so — where does it go?
After the fighting stops, the combatants will have settled into three camps. Let's look at all three in turn.
Incidentally, because the names of holidays are written with an initial capital, we write Fathers Day, not fathers day, regardless of where you put the apostrophe, if any.
Choice 1. Fathers Day: no apostrophe
The argument here is that fathers do not own the day, so no possession is involved. No apostrophe is thus needed. We are describing a day for fathers, not a day belonging to fathers.
Choice 2. Father's Day: an apostrophe before the s
Here the argument is that the day belongs to one specific father (yours presumably). So, because possession is involved, Father's Day needs an apostrophe.
Choice 3. Fathers' Day: an apostrophe after the s
Here the argument is that the day is shared among all fathers collectively. We thus need an apostrophe after the s.
And the winner is...
So, which is it? Well, let me stress that you can make a reasonable case for all three of the choices, so it comes down to whether you think that ownership is involved or not.
In British English there is an increasing tendency to omit apostrophes when for is implied rather than of or belonging to. Thus an increasing number of editors of British English publications are opting for choice one (no apostrophe) arguing that it is a day for fathers, not a day belonging to a father or fathers. US usage, however, is predominantly split between choices two and three.
Having said this, there is still a great deal of uncertainty about the issue and you will certainly continue to see all three forms in use regardless of where you live.
If you agree that Fathers Day doesn't need an apostrophe (and feel free not to agree — this one's still controversial), you will probably agree that these don't need apostrophes either:
- girls school: a school for girls, not owned by girls
- visitors book: a book for visitors, not owned by visitors
- workers canteen: a canteen for workers, not owned by them
Note that in all the cases above the word for is implied, so no apostrophe is needed.
In the following cases, though, of or belonging to is implied, so we do use an apostrophe.
- John's report: the report belonging to John
- Bach's music: the music of Bach
- the team's bus: the bus belonging to the team
A rule of thumb
If an apostrophe implies of or belonging to, keep it. If it implies for, lose it. (Keep in mind, though, that this is a predominantly British- English usage at present.)