The closer to ovulation you get, the darker the line will be. So it makes sense that you'll be ovulating around Friday and I bet the line will be dark by then.
With that said, I'd start having sex now. Sperm can swim up to 5 days after ejaculation in optimal conditions. It stands to reason that if you have sex leading UP to ovulation (tonight through Friday), you'll have a good chance of hitting that egg!