When do you begin to push

I feel my pigs are going to be about 20#'s behind where i want them at fair, when would you start the push on them

If you are sure they are going to be light for your show (you have done the math and weighed and figured out what they are gaining/day) than I would kick them up sooner rather than later.  It is fairly easy to hold a pig (meaning little to no gain/day) it isn't always easy to push them especially if they decide they don't want to eat.  They should be gaining about 2lbs / day give or take.  How big are they now?  When is your show?  What do they have to weigh?  What are you feeding them and how much?