To set the timer for Christmas lights, you'll need to locate the timer settings on your Christmas light controller or app. Once you've found the timer settings, you can set the start and end times for your lights to turn on and off. For example, you could set your lights to turn on at 6 pm and turn off at midnight. You can also adjust the days of the week that the lights are set to turn on and off, so that you can have a different schedule on weekends or holidays.