What time of day do you OPK? Confused!

I am using OPKs for the first time this month. I got a slightly darker line on Thursday, so thought it was on its way, but the line disappeared again. My temperature hasn't gone up so I don't think I have ovulated, but I am well confused now! Glow predicts I ovulate tomorrow, but tests today are negative. I'm now wondering if I've drunk too much liquid or done it at the wrong time, or what... Waaa! Help!