The optimal time to take a nap is around 2:30pm. 12 hours after the time of deepest sleep at night. Our internal clocks naturaly make us sleepy around this time so it should be easier to slip into REM. It also isn't so late that it interferes with sleep at night.

I personally haven't had much luck with getting lucids during naps. Mostly because it takes me to long to fall asleep for WILD and I rarely remember nap dreams for some reason. I hope it works well for you!