"Ah yes, you mean the day I'm desperately trying to erase from my memory," Ianto replies with something very much like his normal sarcasm. He takes a sip of his drink, then, because he desperately wants something to laugh at, something to break the horrible tension of this place he adds, "Did I tell you Spike decided to propose with a skull ring?" He's trying to avoid reminding Jack of that night as much as possible but it's hard not to find that fact a little hilarious.