First matching sock pair arriving on day three

A drawer holds $10$ socks forming $5$ pairs, each pair a distinct colour. On Monday two socks are taken at random from the drawer; on Tuesday two more are taken at random from the $8$ left; on Wednesday two more are taken from the remaining $6$. What is the probability that Monday's and Tuesday's socks are both mismatched but Wednesday's two socks match?

$26/315$. By symmetry of the order in which the three pairs are drawn, this equals the probability that Monday matches and Tuesday and Wednesday don't: $\tfrac{5}{45}\cdot\tfrac{28-4}{28}\cdot\tfrac{15-2}{15}=\tfrac{26}{315}$.