Meaning of 閒日 (cantonese)
Words.hk defines it as:


  名詞
  星期一至五, 唔屬於星期六日或公眾假期嘅日子（量詞：個）
  weekdays: Monday,Tuesday,Wednesday, Thursday or Friday; usually with the restriction of a non-holiday


And they give the example:


  閒日飲茶有折喎！
  haan4 jat2 jam2 caa4 jau5 zit3 wo3!
  Discount for dim-sum on weekdays! 


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閒日 is literally made up of the characters for idle + day. Why weekdays are idle days could only be explained by slow business - because business is would, assumably, be good on the weekends and holidays. But you’d never hear this usage in Mandarin where it just simply means “off day,” i.e.: a day where one doesn’t work.

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Guangzhouhua-Putonghua gives it a different Cantonese definition:


  背集；不逢集的日子。


“Non-market days.”

CCCanto’s definition is also similar:


  free time or off day