What is the correct punctuation of the line "their son williams party"?

Question

is the apostrophy before aor after the s or isnt there one

Answer 1

their son William's party, because William is singular if it was a party for all the Williams then it would be after the s

Answer 2

Their son William's party.  It's his party, he owns it and the 's shows singular ownership.  s' shows ownership for for more than one like the three cats' bowl or ownership for someone whose name ends in s.

Answer 3

William's party (singular, possessive)

-plural doesn't require apost.: sons, daughters

-apost. after 's' is plural possessive: (if they had 2 sons, for ex.) their sons' parties  

Answer 4

William is the son's name and it is his party, so it would be "their son William's party"