This is one of the areas where a great many people seem to slip up grammatically and where another great horde of people become very irate.
The apostrophe has been partnered up with the letter s in a few different roles over the last 500 years, and as with many relationships the goalposts have shifted a little over time.
If the apostrophe appears before the letter s then it may denote that the possessive case is singular, i.e. the boy’s girlfriends. If it appears after the s, then it will most likely be plural, i.e. the girls’ boyfriends.
However, if the name or word itself ends in s (for example, dress or Ozymandias), then you find yourself in muddier waters.
Some will say that an s and apostrophe combination is always better because it makes things clearer (the dress’s sleeves); others will stick to the older method of leaving the s out after any word or name ending with an s, z or x (Davey Jones’ Locker).
Some believe you should add an s if you would pronounce the sound while speaking the words (Ozymandias’s plans).
On the other hand, it is also customary to leave out the second s if the s in the word is already somewhat slurred and z-sounding: for example, Moses’ followers or Jeff Bridges’ beard.
To simplify things a little, any French names ending in s (or x) should always be followed by ‘s when used possessively, e.g. Rabelais’s carnival, although only one s sound is pronounced.
The same goes for Classical names ending with s (such as Mars’ helmet or Socrates’ sandals).
In short, the trend seems to be headed toward all names and nouns ending in s being followed with ‘s when used possessively.
Place names make up a more random kettle of fish. Any logic to whether an apostrophe is used or not is lost to history, so you just have to check for each place: for example, St Albans is in Hertfordshire, but there is a St John’s in Canada; or worse, leafy St James’s Park in London enjoys the s that the hardy Geordies insist St James’ Park in Newcastle does not need.
Even when history repeats itself, it isn’t always sure of the punctuation.