There is a lidl in banbridge. It's on the newry road, opposite the church.
As shesells said it will be very busy on a saturday i'd leave early moring say 7.30-8.00 or leave later on in the afternoon, unless you want to have a look around the other shops as well.
I shop on a monday so i dont have to worry about it been over-crowed or empty shelves. I remeber xmas shopping and never again the lines were all the way to the back of the shop on every check out, you couldn't even get up and down.