When should the lines go dark?

My fertile day is meant to start on Saturday, my OPKS are very very faint, sometimes they don't show up at all (depending on time taken usually twice a day) I haven't had a spike in temperature yet, is this normal considering my 'green' day should start on Saturday? When should be OPKS start getting dark? I don't know if it makes a difference but last month AF was over 3 weeks late