T.O.DAY (Topology Optimization in DAYlight Analysis) is a novel facade design approach that uses topology optimization techniques to maximize daylight penetration for 3D-printed structures. By strategically rearranging the voids within a facade, T.O.DAY could significantly improve the amount of natural light reaching the interior of a building. Additionally, it could reduce reliance on artificial lighting and energy consumption. All these could improve daylighting by introducing daylight analysis into early design stages, allowing for a more holistic design approach. T.O.DAY’s methodology is based on traditional topology optimization, with a key difference: instead of focusing on structural strength, it optimizes daylight metrics. The methodology involves dividing the facade into finite elements and identifying the optimal locations of voids that enhance daylighting. T.O.DAY is a design tool that can lead to sustainable facade design in terms of material efficiency, energy consumption, and daylight improvement, respecting the comfort of building users.