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What is the probability for $3$ siblings' birthdays to fall on the same day of the week every year since the second sibling was born? This has occurred over 100 years before the first sibling was born and is still re-occurring $70$ years after the first sibling was born. Leap years make no difference.

The dates are April $6$, June $15$, and July $27$.

Thank you!

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Once they are born, if the birthdays are all on the same side of February $29$, the pattern (whatever it is) will recur every year. In your example, June $15$ is $70$ days after April $6$. Because $70$ is divisible by $7$, they will fall on the same day. Similarly, July $27$ is $42$ days after June $15$, so they will fall on the same day.

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The probability that any two people will have the same birthday on the same day of the week every year is about 1/10. (If you want a few more decimals, it's about 0.10402.)

The probability that three people have their birthday on the same day of the week every year is about 1/92. (If you want a few more decimals, it's about 0.010820.)

As @Ross Millikan said, once this pattern starts, and everyone is on the same side of February 29, the pattern will always continue forever.

This has the exact math for two people

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