Today is husband’s and son’s birthdays.
Today is husband’s and son’s birthday.
Today are the birthdays of my husband and son.
There are many other permutations of this sentence to confuse me.
There is one husband, one son and only one April 18th. How many birthdays are there?
There must be two birthdays: husband has been having birthdays on April 18 for 41 years. Son has only had a birthday on this date for 13 years.