# Probability for $3$ siblings

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!

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.