Add sugar to boiling water that's been boiled in medium saucepan, stirring until sugar is dissolved. Add lemon zest, and juices. Put into quart mason jar or container and refrigerate until ready to use.
When ready to serve, add 5 cups of cold water to concentrate and stir until sugar syrup is incorporated into the liquid. You can adjust the water depending on your taste. (If you like a stronger taste add less, if you like a weaker taste add more.)
Pour into a pitcher and chill. Serve over ice, and garnish each glass with a fresh lemon slice and a sprig of mint leaves, if desired.