the evil moment/hour/day

The moment, time, or day when one must do something particularly difficult, unpleasant, or stressful. I knew I had to start filing my taxes soon, but I've been doing everything in my power to put off the evil day. Having "the hair of the dog" may make you feel slightly better after a night of heavy drinking, but it's only postponing the evil moment when your body's hangover eventually kicks in.

the evil ˈhour/ˈday/ˈmoment

the time when you have to do something difficult or unpleasant: I’d better go and see the dentist — I can’t put off the evil hour any longer. ♢ I worried for weeks about how I would tell him the bad news but eventually I couldn’t put off the evil day any longer.