Whether or not you are installing outdoor lighting or Christmas lights, the system needs to be protected. Problems caused by rain, sprinklers, or just general mischief are stopped with specialized outlets.
The outlet you should be using should be GFCI (ground fault circuit interrupter) protected. The GFCI outlet may be at that location, upstream of that location, or you may have a GFCI breaker in your main electrical panel.
You can buy a tester for about $5 to check that it is protected. Just plug the tester into the outlet and press the button on the tester. If the power to the outlet stops, then it is GFCI protected. If you test the outlet and you still have power, install a GFCI outlet in that location.
It you are worried that while in use, the rain could enter the outlet, this can be fixed (this is possible since most box covers have doors that must stay open to access electricity). There is an easy fix, however. You can buy a weatherproof box cover that is made for use during rain (or when your sprinklers hit it).
This type of cover is essentially a semi-dome shape with one or two holes at the bottom. The dome shape has enough space for the plug, and the holes at the bottom fit snugly around the cord as it leaves the box. Replacing the old cover with the new one is easy.
When you go to buy the new cover, take note as to whether you will need a vertical or horizontal cover (this makes a difference because you want the cord holes to be on the bottom).
A new cover will cost under $15.
When you get the cover home, turn off the power to the outlet at the main panel. Don’t just trip the GFCI “test” button. This will kill power to the outlets that are downstream of the GFCI, but live power will still flow into the outlet. If your outlet is downstream of a GFCI, you can trip it so long as nobody inside the house decides to reset it.
Getting shocked is no fun. I once got shocked by a 220-volt current, and I made noises I didn’t think I could make. The whole episode took a second or two, but it seemed like slow motion from where I was standing. Anyway, just be sure that the power is off before you start toying with electricity.
Remove the old cover and the foam gasket by unscrewing the single screw in the center. Most new covers will house the outlet inside, so you will have to unscrew the outlet from the box, slip the new cover and foam gasket over and behind the outlet, and screw the whole works back together.
You won’t have to remove the wires from the outlet as you do this, just free the outlet from the box. As you place the box back against the wall, align everything one final time before tightening. On some brands, you will also have to install a standard wall cover inside the new weatherproof cover.
Turn on the power and plug in your lights. You don’t need to be afraid of the rain again.