McGonagall, a very talented Quidditch player, is fouled during the Gryffindor versus Slytherin match which decided the Cup. She suffered a concussion and several broken ribs. From this incident Minerva developed a lifelong passionate desire to see Slytherin defeated in Quidditch (Pm).
This incident took place during McGonagall's final year at Hogwarts, which would be 1953-1954. Since the match decided the outcome of the Cup competition, we'll place it during the second half of the school year, in the spring of 1954.