说明
on Sunday/on Sundays
核心问题
（1）What are you going to do on Sunday?
(2) We stay at home on Sundays.
句(1)中的 on Sunday 与句(2)中的 on Sundays 意思相同吗?
词典解答
on Sundays/Mondays 是“每逢星期日/星期一”的意思：
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Christians go to church on Sundays. 基督教徒每个星期天去教堂做礼拜。
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I never eat breakfast on Sundays. 我星期天从不吃早饭。
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Many people do their week's washing on Mondays. 许多人在周一洗一周的衣服。
on Sunday/Monday 也可以表示“每逢星期日/星期一”的意思：
( On ) Sunday(s) we usually go for a walk. (CGEL) 我们通常在星期日出去散步。
但是,它还可以表示“上星期日/上星期一”(last Sunday/Monday)或“下星期日/下星期一”(next Sunday/Monday):
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I went there on Sunday. 我上星期日去了那儿。
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He'll arrive on Sunday. 他将于下星期日到达。