Updated project metadata. The adaptive immune response is under circadian control, yet, the benefit of this rhythmicity for the organism is unknown. Furthermore, it is not understood why adaptive immune reactions continue to exhibit circadian changes over long periods of time. Using a combination of experimental and mathematical modelling approaches, we show here that dendritic cells (DCs) migrate from the skin to the draining lymph node (LN) in a time-of-day-dependent manner, which provides an enhanced likelihood for functional interactions with T cells. Greater numbers of infiltrating DCs induce rhythmic expression of TNF-α in the draining LN, which enhances ICAM-1 expression in high endothelial venules (HEVs), resulting in lymphocyte infiltration and LN expansion. Icam1 is controlled by binding of the core circadian clock transcription factor BMAL1 to its promoter region, and rhythmic Icam1 expression is lost in mice lacking endothelial cell BMAL1. LN cellularity continues to be different for weeks after the initial time-of-day-dependent challenge, which determines the immune response to vaccinations directed against Hepatitis A virus as well as SARS-CoV-2, as evidenced by rhythmic T cell reactions, germinal center formation and antibody production. Our results provide the mechanistic understanding of the time-of-day dependent development and maintenance of an adaptive immune response, demonstrating its dependency upon the timing of the initial challenge and the interactions of rhythmicity in multiple parameters. This provides a strategy for using time-of-day to optimize vaccination regimes.