April 19
Although the Columbine massacre took place on April 20, 1999, it was originally planned for April 19.
April 19 - The Original Date
Eric Harris and Dylan Klebold originally planned their massacre for April 19, 1999.
The proof
Table carving
During the investigation on 4/20/99, the words "WRATH" "V" and "4-19-99" along with "Doom, revenge, and terrorism" were found scratched into a table in Clement Park (Evidence, p.11764):
This alone isn't concrete proof, but it becomes obvious that 4/19 had meaning here when you look at the other pieces of evidence.
Also, note that this was found on 4-20-99 during the investigation, so it wasn't fabricated by a random person after the fact. There are photos floating around of a similar carving, but this is not the same table nor the same carving.
Basement tape reference
In one of the basement tapes, Eric Harris is in the driver's seat of a car, Dylan Klebold is in the passenger's seat, holding the camera. "We are on our way to get the rest of our gear," they say. They then tell the camera it is "Monday, April 11th". It's daytime.
Then they say "7 and 1/3 more days." That could only add up to the morning of April 19th.
Why did they change the date to the 20th?
The short answer is nobody knows.
It's speculated that they changed the date because Manes forgot to get them the ammo they requested, but that can't be since by April 12, three days before they asked him to buy them ammo, they made a comment about the date being set for the 20th.
On April 12, 1999, in a basement tape clip, Eric states, "The apocalypse is coming and it’s starting in eight days."
April 12+8 days=April 20.
When did they change the date to the 20th?
The planned date had to have been changed after they filmed the segment on the 11th because by the 12th, Eric said the shooting would begin in 8 days (the 20th).