On October 4, 1963, Lee Harvey Oswald was looking for work, and Mrs. Ruth Hyde Paine, a friend of the Oswalds, said in later testimony:

He was getting pretty discouraged. I called the Texas School Book Depository and asked whether they were employing at this time, whether they had an opening.

Oswald applied for (and got) a job the next day.

This was long before the luncheon site for JFK was even picked, let alone the route for the motorcade. Plans were in motion for a visit: a Kennedy trip to Texas had been discussed for nearly a year (the Democratic Party in Texas was struggling) and a firm decision of a late November trip was made on June 5 (while the President, Vice President, and Governor Connally of Texas were at the Cortez Hotel in El Paso).

In November 4, Gerald Behn (in charge of the White House detail) was asked to pick three potential sites for a luncheon that JFK would speak at.

Market Hall was not available on the planned date (November 22).

The Women's Building at the State Fair Grounds was deemed to have issues with the building itself (including exposed conduits in the ceiling).

Trade Mart, the third option, was a new building and deemed acceptable. The decision to have the luncheon there was made final on November 14.

Two Dallas papers announced the motorcade; the Trade Mart was mentioned on November 15, and then more explicitly the next day it was reported the next day the President would "loop through the downtown area, probably on Main Street, en route from Dallas Love Field". On November 19, the exact route was laid out in the Times-Herald:

From the airport, the President's party will proceed to Mockingbird Lane to Lemmon and then to Turtle Creek, turning south to Cedar Springs. The motorcade will then pass through downtown on Harwood and then west on Main, turning back to Elm at Houston and then out Stemmons Freeway to the Trade Mart.

The November 16 reference would be enough for Lee Harvey Oswald to realize the Book Depository would be along the route, and of course November 19 made it entirely explicit. The fact he had recently obtained a job downtown was simply an opportunity he used, not a plan pre-made (as the actual motorcade plan was a month away, there was no way to have pre-knowledge).

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Further details on the plans for the motorcade are in the Warren report.

I've also written in the past about the grassy knoll and the "three tramps".