On its own
昼 is literary. In speech and modern writing it almost only appears in compounds, especially
昼夜 (zhòuyè, day and night),
白昼 (báizhòu, broad daylight), and
昼伏夜出 (zhòu fú yè chū, 'hide by day and emerge by night' — said of nocturnal animals or covert activity). Everyday 'daytime' is
白天 (báitiān).