Indoor extension cords don't belong outside.

No, it is not safe.

Even if it doesn’t rain, moisture happens. Dew forms overnight. The ground can be damp. Sprinklers might run. And indoor cords aren’t built to handle that. The insulation is thinner, the plugs lack weatherproof covers, and they aren’t grounded for outdoor use.

A few hours feels harmless, but that’s exactly where accidents start. A cracked jacket, a little condensation, and you’ve got a shock risk instead of a light string.

Buy the outdoor-rated cord. It costs a few bucks more and gives you actual peace of mind.