It can be extremely difficult to perfectly track a period. Even when you do get the signs and symptoms, it sometimes doesn't come until a few days later--or sometimes even a week.

I've always tracked my period, but that doesn't mean it didn't catch me by surprise sometimes. Periods can be irregular.

That's exactly when someone would spray tan. They may want to a tan when showing skin in summer clothes, and not want to be risking skin cancer, and/or they could be naturally really pale and unable to tan.