Click on the front door of your active Sim's home, and the option should appear to "hang holiday lights". Though I had a home where it showed when I didn't need it, but wasn't there when I did. They look very cool once hung - you get a choice of styles and colors to use. =}
They are also harder to see in the daylight then in the daytime, and it could take some sim playtime for them to show up. Many times the outdoor holiday lights don't show up until the second day of that season.