Does plan b work if taken the day ovulation starts?

Yes: It works if you take it within 72 hrs of unprotected sex irrespective of the cycle date.

It can be taken at: any point during a woman's menstrual cycle. Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but is less effective as time passes.