Let’s start with when:
Any time you refer to your family in writing! Some specific occasions you pluralize your surname are when writing your return address and when spelling your name on your mailbox or front door.
You might address other families by last name when they come to your home or when you send them a social invitation. How do you write the plural forms of their names?
What not to do:
Never use an apostrophe to create a plural! Apostrophes create possessives; they never create plurals. See More . . .