A trading day is any day you place a trade. Days you do not place a trade, referred to as calendar days, do not count.
If you fail an evaluation, don't worry, it happens. The important part is not to give up and learn from it.
Suppose you are doing well in your Fast Track evaluation but didn't hit your profit target within the maximum allowed days. In that case, you can "roll" your evaluation into a 1 step evaluation of the same size at a discounted rate. You will see a notice on your trader dashboard with information on how to do this.
Note: Fast Track evals do not support resets