Do you say jiff's yesterday lecture or jiff's yesterday's lecture?

Answer 1

Neither one sounds correct in basic English. Instead, one would probably say, "the lecture Jiff gave yesterday," or "yesterday's lecture, which was delivered by Jiff."

Answer 2

You should say "jiff's yesterday's lecture" because the apostrophe followed by an 's' indicates possession, showing that the lecture belongs to jiff and occurred yesterday.