How crazy?!

Your first day of your period is day 1. So day 7/8 is about 1 week after your first day of your period. About 14 days after ovulation, you'll get your period again. Since the day of ovulation doesn't always fall on the same day for all women, they base this on the shortest normal cycle of 21 days. In the days leading up to ovulation, you're also fertile because sperm can survive in a woman's body for a few days. How long depends on the conditions in the woman and the quality of the sperm. You'll get a positive test a few days before ovulation due to the hormones you release. And it will stay positive until the egg is released.
Usually, if your cycle is longer than 21 days and you test on day 7, you'll have some negative tests before it turns positive. The more you know about your average cycle, the fewer tests you need to track ovulation.