Having sex multiple times and taking one Plan B.

Did you end up getting pregnant??(:

Plan B is not as effective as regular hormonal birth control. It's simply worth a try in an emergency. I deal with women who become pregnant despite using it.
There's even a page on the ec princeton website, about emergency contraception, which shows that of women who used Plan B on their day of ovulation, there were as many pregnancies as those who did not use any emergency contraception.

Even if it's not your ovulation day, it doesn't always work, & should definitely not be used as contraception that is planned ahead.

It's also a huge dose of artificial hormone for your body to deal with. It can cause lots of hormonal side effects, for weeks or months in some women.

In the situation you gave us, if you ovulate on the first day of sex, then don't take the Plan B pill until the Tuesday, the Plan B pill can't do anything, because you'll have already past your ovulation.
Or you might be in plenty of time before ovulation, but Plan B happens not to manage to delay your ovulation this time. That can happen.

The only emergency contraceptive pill proven can prevent pregnancy on your day of ovulation is Ella. You can see that on the Manufacturer's website. Even Ella is not guaranteed.