Organisms across all kingdoms of life have evolved circadian clocks, genetically-encoded timing systems with ≈24-hr period. It is thought that the role of circadian clocks is to generate daily rhythms in biological function and to align these (internal) rhythms with the (external) day-night cycle. However, it is poorly understood how circadian rhythms align to dawn and dusk as their timing changes from season to season. In my thesis, I addressed this question in two different biological systems. In work described in Chapter 2, I established the cyanobacterium Synechoccocus elongatus 7942 as a model of seasonal adaptation of circadian rhythms. This chapter reports high-throughput methods to monitor clock function of cyanobacteria grown in programmable light-dark environments (e.g., mimicking day lengths in different seasons). Further, it describes the in vitro reconstitution of the seasonal clock response using metabolites to mimic the effects of day and night on the core clock comprised of the Kai proteins. A major conclusion of this work is that the cyanobacterial clock tracks the middle of the day in conditions with varying day length. The chapter concludes with a generalizable mathematical framework describing how a circadian clock adapts to light-dark cycles. I took a different approach in the study covered in Chapter 3, using county-level records of Twitter activity in the United States to examine how the timing of daily rhythms in human behavior varies region to region and season to season. The timing of tweeting patterns is significantly affected by seasonal changes in the social calendar, such as public and school holidays, while the direct effect of seasonal changes in day length appears to be less pronounced. Circadian biologists are increasingly aiming to study biological rhythms in ecologically-relevant conditions. Approaches range from developing tools to mimic natural environments in the lab to monitoring daily rhythms in animals living in their natural habitats. Studies described in this thesis add to this emerging discussion from both sides of the spectrum, demonstrating that cyanobacteria are a powerful platform to probe the biochemical mechanism of clock alignment to the day-night cycle and that social media activity records can reveal how daily patterns of human activity in the real world are affected by geographic, seasonal and social factors.