First time opk user plz help!

So I just started doing opks yesterday and Ive read that some woman do it twice a day. I took one yesterday around 1:30pm and then the same again today. According to glow my fertile window will start on Sunday. Well what I don't understand is if I am supposed to start my fertile window on Sunday why was my opk yesterday darker than it was today? I didn't have an lh surge because the line wasn't close to the color of the control line but why is it lighter today shouldn't it be getting darker as I get closer to my fertile window and when should I start testing twice a day?Please help!