I love Christmas lights, and it turns out people (rich people, at least) have been loving electric lights longer than one might have suspected... here's a nice little number from General Electric, vintage 1904:
I include this under the rubric of ads we would not see today given our contemporary concerns about safety. I'm thinking I would not let my four-year-old anywhere near a string of wiring manufactured in 1904, even if it was brand-new at the time. A "festoon" of eight sockets and ten "lamps" cost $5.00. For the sake of reference, the average weekly wage in the United States at this time for a worker in the building trades was $17.87 (assuming a full work week averaging 48.3 hours). The average teacher earned $6.30, and the average worker in medical and health services earned $4.92. 
You can see this image, other advertising brochures and even the text of these early ads on the following page of an excellent Web site called "Old Christmas Tree Lights": http://www.oldchristmastreelights.com/history_2.htm. Enjoy!
 Facts and Figures, Income and Prices 1900-1999, US Diplomatic Mission to Germany: http://usa.usembassy.de/etexts/his/e_prices1.htm.