五月五日 (wǔ yuè wǔ rì) traditionally refers to Duanwu Festival (端午节 Duānwǔ Jié, Dragon Boat Festival), held on the 5th day of the 5th lunar month. In classical writing 五日 alone could already evoke the festival.
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More formal/literary than the everyday 五天 (wǔ tiān, five days). Use 五日 in dates, schedules, and written notices; use 五天 in spoken duration.
