When Was the Conspiracy to Assassinate Archduke Franz Ferdinand Hatched?

Answer: April 1914

That month Gavrilo Princip was in Belgrade, where he associated with a number of Serbian students in town cafes and conceived a plan for the assassination of Archduke Franz Ferdinand. He shared the plan with his acquaintance Nedeljko Čabrinović, also in Belgrade, who held similar views and agreed at once to participate in the attempt.

The Conspirators in Court, Princip Circled

Attempts on the archduke's life were a frequent topic of conversation in the circles in which Princip and Čabrinović moved, as the archduke seen as a dangerous enemy of the Serbian people.

Princip and Čabrinović desired at first to procure the necessary bombs and weapons from Serbian Major Milan Pribićević or from the Narodna Odbrana, [the Black Hand] as they lacked the money to purchase the weapons. Since both Pribićević and Živojin Dačić, a leading member of the Black Hand, were absent from Belgrade, they then tried to get the weapons from their acquaintance Milan Ciganović, a former Komitadji [nationalist rebels originally working against the Ottomans] currently working for the state railways. 

Princip contacted Ciganović though a friend and discussed the assassination plan with him. Ciganović endorsed the plan and indicated he would consider providing weapons. Čabrinović also talked with Ciganović about the weapons.

At Easter Princip took Trifko Grabež, also in Belgrade, into his confidence. In his later confession, Grabež admitted his willingness to take part in the attempt. In the following weeks Princip repeatedly discussed the plans with Ciganović, who meanwhile had reached an understanding with his close friend Serbian Major Voja Tankosić to provide the Browning pistols Princip used on 28 June to kill the archduke and his wife, Sophie. 

Sources: Austrian Court District of Sarajevo Record