Why is Jonas is in such a hurry to enact the plan?

In chapter 20, the Giver and Jonas carefully plan Jonas's escape on the day of the Ceremony of Twelve. Their initial plan was for Jonas to leave a note with his parents explaining that he went for an early morning bike ride. When Jonas's parents left to attend the Ceremony of Twelve, Jonas would leave his bike by the river and hide in the Annex with the Giver. The Giver would then call for a driver and send him on an errand when he arrived, which would allow Jonas time to sneak into the storage compartment of the vehicle. When the driver returns, he would unknowingly escort Jonas out of the community. When Jonas's absence becomes noticeable, the citizens would find his bike by the river and assume that he drowned in an accident. The Giver would then help the citizens cope with Jonas's difficult, loose memories.
Unfortunately, the entire plan changes and Jonas is forced to flee the night after devising his escape plan. In chapter 21, Jonas learns that Gabriel will be released on the following morning. In order to save Gabriel's life, Jonas takes the infant and flees the community the night before Gabriel's release. Fortunately, Jonas narrowly escapes the community and begins his arduous journey to Elsewhere.