Why are you going to ask me about my day and then when I start to explain my day you interrupt me or ignore me? IF you really don't care then don't ask. Don't be rude about it.
I have found that it seems to be some sort of weird automatic question some ask. I started giving nonsensical answers to weed out those who are genuinely asking. "How was your day?" "Eh, well, cabbage doesn't grow under stones." About 90% of the time, no one reacts with confusion.