TYPICAL ERROR We celebrated Mum's and Dad's anniversary yesterday.
CORRECTION We celebrated Mum and Dad's anniversary yesterday.
EXPLANATION Notice that in the example above, the anniversary is a joint celebration and is 'owned' by both Mum and Dad. In such cases, possession should be indicated by using an apostrophe only after the last owner. If possession is not joint, both words should show possession by the use of an apostrophe in each, for example, we should write:
Mum's and Dad's clothes are in these suitcases.
rather than
Mum and Dad's clothes are in these suitcases.
because they would each have their own clothes.