What is the hat rule for ladies?

Excuse me, could you possibly clarify what you mean by "the hat rule for ladies"? Is this perhaps a reference to a specific social custom or etiquette guideline surrounding the wearing of hats by women in a certain context or setting? I'm curious to understand if there's a particular occasion, event, or tradition that this "hat rule" pertains to. Additionally, are there any cultural or historical nuances associated with this rule that you'd like to highlight? Your explanation would be greatly appreciated.