For the record: The lackluster Flash Gordon reincarnation, which was on SciFi last season but now has moved to Ion, is on at 7/6 C on Fridays. Not Friday's. Fridays. I know it's difficult, Ion marketing/advertising people, but with practice you too can learn to put the letter s at the end of a word without an apostrophe.
I put it to my students thusly: The apostrophe-s indicates either possession or contraction:
It is a beautiful day in the neighborhood = It's a beautiful day in the neighborhood = Correct!
The ball belongs to Spot = It is Spot's ball = Correct!
Members of the Nelson family are coming for dinner = The Nelson's are coming for dinner = Incorrect!
The lackluster Flash Gordon is on every Friday at 7 = The lackluster Flash Gordon is on Friday's at 7:00 = Incorrect! (Except for the lackluster part.)
So as I told the class, if you can't rephrase the sentence to indicate a contraction or possessive (and still have the sentence make sense!), then you need to ditch the apostrophe.
We will save for another day the matter of forming possessives when the word ends in s already!
Addendum A: I do wonder, however, about the mailbox mentioned above. It is, I suppose, possible that the labelers of said box intended to indicate that it belongs to Anderson, in which case Anderson's would be correct. But I rather suspect it's meant to indicate that the box receives mail for members of the Anderson household, in which case Andersons is what you need. Of course, you can save time, confusion, and a sticky metal letter if you go with Anderson and hope the letter carrier can figure it out from there.
Addendum B: Some years ago I worked for a company called The Webb Company in St. Paul, Minnesota. It no longer exists. The company, I mean. Anyhow, some time after I left there I communicated to a former co-worker that I would be meeting a friend of his who also had worked at Webb but had left shortly before I joined. My friend told me not to be alarmed when the other fellow referred to the place as Webbs, as my friend rendered it in his letter to me. And sure enough, when I introduced myself to the other fellow, mentioning our mutual friend, he in turn introduced me to his wife and said that I too had worked at Webbs...except that, as I later expressed to my friend, I think it was in fact more properly rendered Webb's. I have often noticed a local tendency to incorrectly render various business names as
possessives. One of the most common around here is the John Morrell meatpacking plant, which is universally referred to ('round here) as Morrell's...even though none of their signage or labeling has ever, to my knowledge, said anything except John Morrell or Morrell. I don't know why this should be, unless it goes back to a time when many if not businesses carried the proprietor's name: Bob's Grocery, Foley's Garage, etc. (Sure, names like that are still around, but my guess is there are fewer than in bygone times.)