What a perfect topic! Exactly how I feel... "wasted day"...
In my case, I get anxious and nervous and ill humoured (really), already in the morning, if my perspectives of playing the piano during that particular day are not promising. If, say, I have people visiting me in my apartment on a Saturday. If they stay too much and I start to foresse that I will not have at least one hour to play before 10 PM (I can only play until 10 PM, because of neighbors, the silence rule and stuff), I really have a bad time trying to control myself. My fiancée is the first one to notice that I am getting a bit crazy. She tells me I'm a bad host haha...
Vacation is also a problem...
Fortunately, though, I need no more than one hour or so to feel accomplished for that day. It's not that I won't practice much more than that, if I can. But one hour will do, as for making a day count.
Cheers,
Vinicius.