the SOURCE WAR AND PEACE
"And the fete at the English ambassador's? Today is Wednesday.
"S" is of possession, means should ambassador followed by a noun.
Where is the noun?
The possessive case of a person's name, title or occupation is often used to designate their home or place of business.
There's a party tonight at Mary's.
I have an appointment at Dr. Fisher's.
Mrs. Schmidt is over at the Chairman's; she should be back in half an hour or so.