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Term used by women (or men) to politely inform men that they (or the subject) currently are in their menstrual period.
Do you want to go clubbing?
No, sorry. It's lady's day. I'm not feeling that good.

Do you want to buy me icecream.
Lady's day.

I'm really horny. Let's do it here.
No. It's lady's day.

"Don't you just hate lady's day." said Jack.
"Yes," said John, "Wack-off bonanza!"
by Kate McInnes October 02, 2004