I've noticed Siri misinterpreting words pretty easily, too, but most things that I use her for work well. Setting timers, setting alarms, starting/stopping navigation. All of those work well. Where she fails miserably is starting specific songs or artists.
I think pronunciation is a big factor. Have you tried over-pronouncing "day". Humans do tend to get lazy with our speech, and we expect technology to magically understand our ramblings.
"Day" "Living" "Room" -- try pronouncing each word very carefully a few times. Each successful hit will train Siri to recognize your words more accurately.