"Foxes"?? ':) Well, "time"="it", so "do" cannot work in the sentence. In fact "do better" doesn't make any sense in the sentences. "What time is convenient to you?" (spelling!) "What time works better for you?" "What time suits you better?" You can also say, "What time's good for you?" if it's to a friend. "Convenient" sounds formal. "How are you fixed this Saturday?" is possible but I don't use it. Maybe it's a regionalism. Anyway, "fixed" refers to any arrangements or appointments someone may have. You are asking whether the other person has already arranged their plans for Saturday. By the way, "sth" as an abbreviation for "something" only exists in a dictionary.