"To save one's best Sunday Bonnet, in the garden, one might go about in a hat, and in the bosom of one's family wear a pinafore to save a new dress; but in the same way that one did not go into the drawing-room with a pinafore on, one did not, in those days, pay visits in a hat: and to go to church in one would have been thought irreverent."
A bit later she continues: "Sunday Bonnet meant that childish ways were done with, and that one had attained the age of reason. Like a barrister's wig it imposed seriousness on the wearer, who had to live up to it." I think of the Irving Berlin song:
In your Easter bonnet, with all the frills upon it,
You'll be the grandest lady in the Easter parade.
Here's another contemporary bonnet I found:
This child should sue for abuse.