Today signs are seldom handpainted. They are either digitally produced by a print shop or put together by a minimum-wage employee. In the first case, the print shop takes whatever the customer wants and prints it, often without looking at it and certainly without editing it. And few store owners run out and edit the sign board their teenage helper put together advertising today's special. That's why you'll see: Taco's 2 for $1 or "Special on tomato's"
The rules of the apostrophe are actually fairly simple.
- Use for contractions (I'm for I am; you're for you are; etc.) The apostrophe substitutes for the missing letter or letters.
- Use for possessives with nouns representing human beings and animals. The cat's tail, the man's new job, Sharon's coat. If the noun ends in s and is plural, place the apostrophe after the s: the boys' father (several boys) as opposed to the boy's father (one boy). If the noun ends in s and is singular, use 's after the s: Jesus's disciples.
- Use for the plural of symbols: He got all A's on his report card. $'s are less used today.
- Don't use the apostrophe with possessive adjectives or pronouns: This is mine, that is hers (not her's). The cat licked its tail.
- Don't use the apostrophe to make plurals of acronyms but do use with contractions. My PC's broken. He owns three PCs.
This kind of attention to detail is rapidly separating the better writer from the less careful. And it preserves some nuances in the language, always a good thing to my mind.