Here are some tips from a professional photographer on taking photos at a Christmas lights display.
TURN OFF THE AUTOMATIC FLASH
Using the flash might make your subject bright, but it also makes the background black.
LIGHT YOUR SUBJECTS
Try to place your subjects where they are lit by other external lights around them or from behind the photographer. This works well in a tunnel of lights or when walking among many lights. If that doesn’t exist, enlist help from others. Use the flashlight apps on a second or third smartphone to illuminate your subjects’ faces. Don’t use a real flashlight. It’s too focused on one place.
FOCUS ON FACES
Make sure your phone is focused on the right subject. With most phones, you can touch the face on the screen to tell the camera app where the focus should be. Don’t let the camera decide. Pressing on your subject will brighten your picture.