OK, I give you a B+ for that exchange. Handled pretty well but how could you hope she won't think the only reason you decided not to go was for her when you tell her specifically you weren't going because "you didn't want to ruin her day"?
Still, not bad, she got some space, you honored her request. Slightly cooler would have been to just say you have the bathroom project to compete and left it that.
Depending on how it went down, the "I didn't want to ruin your day" comment to her could be perceived as antagonistic, or weak, or even pursueing in a "see I did exactly what you asked and I am now pointing it out sort of way".
All and all though, good move, handled pretty well on your part. Good luck. Don't over excited or go flying her way, if she comes home in a better mood and wants to talk about the day today. Keep calm, pleasant and chalk it up to one small victory. Good luck.
Me 44 She 46 S13 D9 M18 T23 3 years DB'ing Successfully busted