Three friends sharing a weekday of birth

Three friends each have a birth weekday that is uniform over the $7$ days and independent of the others. What is the probability that at least two of them share a birth weekday?

$\tfrac{19}{49}$. Work with the complement: all three differ with probability $\tfrac{7\cdot6\cdot5}{7^3}=\tfrac{30}{49}$, so a shared weekday has probability $1-\tfrac{30}{49}=\tfrac{19}{49}$.