QUESTION:
Is there an exact time you need to launch a spacecraft to get it to Mars
by a certain date? How do you make it land on on July 4th even if the
liftoff date changes by more than a week? 

ANSWER from Cheick Diarra on November 18, 1996:
There is no exact launch time, but there is a window during which one can 
get a spacecraft to Mars by a certain arrival period. The exact date of 
arrival is always tweaked using the propulsive maneuvers scheduled 
throughout the mission. If Pathfinder lifts off a week late, the errors 
due to the launch vehicle could make it get to Mars early.  

In any case chances are it would not be at the exact desired time or date. 
In that case, the spacecraft is made to go faster or slower depending 
on the scenario. 

ANSWER from Dave Spencer:
The July 4 arrival date is maintained by increasing the launch energy
throughout the launch period; the Delta II gave us a higher velocity on the
December 4 launch than it would have if we had launched on Dec 2. The
arrival date was selected based upon a mixture of patriotism and favorable
orbital mechanics--the optimal arrival date was in early July anyway, so why
not pick the 4th!

-Dave Spencer, Mars Pathfinder Trajectory and Navigation Team Member