Physicist: There are 365.25 days per year (on average), there are 31 days in August, and 28.25 days in February (on average). The “.25″ isn’t exact, but the last time that the “leap year every four years” rule wasn’t used was 1900.
The chance that one person’s birthday is not in February or August is 
All of this assumes that the probability of being born on any one day of the year is equal to any other, which is nearly true.
