I take mine for a short stroll round the block after 9.30pm, it was after 11 last night and it was lovely and cool. I keep the windows and blinds closed all day to keep the heat out, which certainly helps, then open them in the late evening for a couple of hours to let the cool air in.
I find a cooling coat is brilliant for evening walks when it's cooled right down, but is still muggy. They feel so cool underneath, and you can take cold water in a thermos flask if you need to "top up" when you're out for longer and the water evaporates. It's one of my absolute best doggy buys.