When pigs fly is said when you think that there is no chance at all of something happening:

For example:

“I think I’ll have finished this paperwork by tomorrow. “Ah, okay. When pigs fly! You’re always late.”

” I might wake up early to clean my room.” “Yeah, right, you’ll do that when pigs fly.”

I’ve already told you that I will learn to dance when pigs (can) fly. Don’t count on that. I have two left feet. People laugh a lot when I give it a try.

Tradução: no dia de são nunca, algo que é muito improvável ou impossível de acontecer, usado para zombar de ambições exageradas ou para responder a algo que se considera impossível.