Lillian lilac tree
You could have a leaf spot disease or it is possible that the damaged leaves froze this spring and you are just seeing it now. Here is a link to the leaf diseases: http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/management-of-leaf-spot-diseases/#bacte... I would remove the discolored leaves and dispose of them. Make sure you clean the area completely this fall. Take good care of your lilac tree and make sure it gets watered when the fall droughty period starts.