It's not an issue if both people acknowledge they had a bad day.
When you know someone has had a bad day you tend to give them some room to sort out things as oppose to making any demands.
Sometimes the best thing to do is for both people to either do their own thing (hobby), exercise, a walk, read, see a movie, or do whatever tends to help them feel better or if there is something they enjoy doing together that takes their mind off everything else.
Essentially both people need a "time out" or "distraction" from their bad day. No one should expect to get a "free pass" to be rude.
Sometimes the best thing you can do is stay apart a few hours.