"Ooooh, it was dreadful! It happened right in here. I died in this very stall. I remember it so well ..."
-- Myrtle Warren
Myrtle Warren is being teased by Olive Hornby and flees into a second floor bathroom to sulk. She hears a male voice — that of Tom Riddle opening the Chamber of Secrets — and steps out to tell him off for being in the girls’ bathroom. Instead she comes face to face with the Basilisk and dies. Her death almost results in the closing of the school.
Harry viewed the events of June 13th via the diary. From the conversations he overheard, it would seem that Myrtle's death was very recent, possibly the day before.