摘要：If you want to avoid the crowds, it's best to come on a weekday. （如果你想避开人群，最好在工作日来。）[柯林斯高级学者辞典]

Sunday用介词on, 同样weekday也是如此，如

【例句1】If you want to avoid the crowds, it's best to come on a weekday. （如果你想避开人群，最好在工作日来。）[柯林斯高级学者辞典]

注意weekday是可数名词，不能裸单。Sunday是专有名词，也是不可数名词，可以没限定词。

复数版的介词与单数版是一样的(从单切换到复，介词再斟酌，也就太辛苦啦)，如

【例句2】On weekdays I'm usually in bed by ten o'clock. （平日里，我通常在十点前就上床睡觉了。）[剑桥高级学者辞典]

【例句3】The centre is open from 9 a.m. to 6 p.m. on weekdays. （该中心在工作日从上午9点开放到 下午 6点。） [牛津高级学者辞典]

in a weekday极少见到。

来源：梳美教育