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.
what is the calculation for P31 =.41%
@dan kulavi
It says P=83.78% so P^31 would simply be 83.73^31 (percent) which is .41%
That’s a suspiciously specific question. I can imagine it arising from someone polling 31 people about their birth months and noticing that two months are missing and wondering how weirded out they should feel. So maybe the underlying question here is “What’s the probability that two months are missing?” That’s a lot larger than the probability that two specified months are missing. (Cf. “p-fishing” and the Bonferroni correction.)