October 4

Today on the fourth of October:

1925: Minerva McGonagall is born.
1984: Will Theakston, who played the Slytherin Chaser Terence Higgs in the first film, is born.

On October 3rd:

1997: Ron becomes increasingly irritable (partly due to his possession of Salazar Slytherin's Locket) and around this time leaves Harry and Hermione to search for Lord Voldemort's Horcruxes on their own.
1978: Christian Coulson, the actor who played Tom Marvolo Riddle in the film adaptation of Harry Potter and the Chamber of Secrets, is born.

On October 2nd:

1991: The Gryffindor First years had an Astronomy class at midnight, on the top of the Astronomy Tower.
1997: Ron becomes increasingly irritable (partly due to his possession of Salazar Slytherin's Locket) and around this time leaves Harry and Hermione to search for Lord Voldemort's Horcruxes on their own.