Sally 你好，
日期 rìqī is the word for "date". For instance 今天的日期是什么？ What is the date today?
日 rì is used in the date to express the day of the month. For instance: Today is Apr 27th: 今天是四月二十七日。
日 rì is also composed with 星期 (week) to form the word "sunday": 星期日， which can also be written 星期天。
Best regards,
天翔
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