Is it fathers' or father's

If you want to abbreviate "father is" use father's

eg: "My father's coming home at 8."

If you are talking about something belonging to one father it's also father's

eg: "That's my father's car."

You use fathers' when you want to make it clear that you are talking about something belonging to more than one father.

eg: "Our team went through hard times but our fathers' support got us through it."