'When' is a difficult thing to determine, as what happened at Columbine High School
was the end result of several months worth of prior planning and suspicious incidents involving the shooters. According to the dates on journals
and video tapes
Denver police found during investigations, 'when' started at least a year prior to the shootings. Police reports
reflect the same, calling out brushes both boys had with the law as early as January, 1998. And the roots of the rage that enabled them to carry out such a horrible plan were in place long before that.
April 20, 1999
is the date when Eric Harris
and Dylan Klebold
attacked their school. However, the narrative timelines
linked here begins in 1993 -- when Eric Harris moved to Littleton. The timelines contain an explicit retelling of the actual shootings; it's a very long and thorough study of the event though by no means exhaustive.
In regards to why they chose to do what they did when they did it, there has been a lot of debate and no solid answers.
Important dates that have been bandied about by the theorists: the Oklahoma City bombing (courtesy Tim McVeigh, ex-military, April 19th, 1995). The Branch Davidian stand-off in Waco (April 19, 1993). Hitler's birthday (April 20, 1889). There's the classic reference to pot-smoking (4:20) of course. These and other theories have been brought up in public forums and on the news but the real reasoning behind the "When" of the shootings has died with Eric Harris and Dylan Klebold in Columbine's library.
Things the two gunmen wrote in their journals
that were confiscated and neighbor Bill Konen's account
of hearing them up late on Sunday the 18th grinding glass led to some speculation that the shooters weren't ready by Monday the 19th so they waited till Tuesday. However, they had 95 bombs
between them, as well as several knives and four guns when they entered Columbine; they weren't exactly short on firepower. There might be more information on the Basement Tapes
regarding why they chose 4-20, but that bit of evidence is still being kept under wraps by Jefferson County.
On Monday the 19th, Eric Harris asked Mark Manes to buy him ammunition
for the TEC-9, telling him that he'd be using it the next day. That same day Dylan detailed out specific plans
in his dayplanner
for the assault they were planning on the school.
The shooters opened fired and the killing started on Tuesday morning, April 20th, at 11:19 AM.
It was just 17 days before graduation.