Day and night; continuously or constantly, without stopping (used like 'around the clock').
試験期間中は日夜勉強した。
During the exam period I studied day and night.
その会社は日夜を問わず顧客サポートを提供している。
That company provides customer support around the clock.
彼は日夜を惜しんで新しい発明に取り組んでいる。
He is working on a new invention, sparing neither day nor night.