Is it safe to have indoor/outdoor Christmas lights turned on outside when it's raining?

They are plugged into an outdoor extension cord ~ which is plugged into a timer ~ which is plugged into an outdoor outlet.

Thanks!

Yes, it is safe for the lights to be on. However all of the other devices hooked up to the circuit need to be rated or listed for outside conditions.

As long as the outdoor outlet is GFI protected.Could be a GFI receptacle, you will see a test and reset button, or could be a GFI circuit breaker, which will also have a test button.