Using apostrophes to show possession in regular plural nouns
This sign should read: Ladies’ toilets
The plural of lady is ladies. You add an apostrophe after the ‘s’ to show that the toilets belong to all of the ladies.
To summarise the rule for general plurals:
When the possessor is a regular plural (i.e. ends in an ‘s’), the apostrophe comes after that ‘s’.
Hope this helps,
Laura