Combine sugar, water and lemon zest in a small saucepan and bring to a boil. Simmer over moderately low heat, stirring constantly, until the sugar dissolves completely.
Pour through a fine sieve into a medium bowl. Discard solids.
Let cool to room temperature, then stir in fresh lemon juice.
Make this ahead if time and store in a sealed jar or bottle in the refrigerator. The syrup keeps indefinitely and you'll have it on hand whenever the mood strikes you.