Wiktionary
vb. (context idiomatic English) to seriously regret one's actions.
Usage examples of "rue the day".
If these be indeed Don Tello and his men, then we must bide where we are, and do what we can to make them rue the day that they found us in their path.
Mr O'Brain was about to rue the day he ignored the threat of the Pursley Brig.
UM #1: (And) he will rue the day he comes here and agitates, because the day he marches in the Saint-Whoever Parade is the day all the white folks put their goddamn internecine and intramural differences aside and unite.
If you don't heed my word, you'll rue the day you failed to listen to the gypsy woman!
Well two could play that game, and he'd rue the day he left the other one just lying about.
He would be better prepared the next time, and she would rue the day she had sought to challenge Katamay Rey!
You may rue the day they burned down your house and brought you back, but this much I know: someone else is going to rue it even more.
I only hoped that my father would rue the day he let his infatuation overwhelm common sense.