I wouldn't be too worried about Christmas lights as a cause of greenhouse emissions.
Whilst individual light displays can use quite a bit of power when operating, they are generally on for no more than about 2% of the time over the year (assuming 8:45pm switch on and off at 1am which is the generally accepted practice in Hobart - I assume it's similar elsewhere) and most houses don't have extensive displays.
Bottom line is it doesn't have a noticeable impact on power demand overall. Sure, there is a bit of a spike in demand when the sun goes down but that happens every day of the year. It doesn't get noticeably bigger in December.
As for the maths, fairy lights typically have 0.1875 Watt bulbs. Put up 10,000 of them and you've got a decent size display but the power use, including transformer losses, is no more than about 2500 Watts. Run that for 4 hours every night from 1st December to 5th January (12 days of Christmas...) and it's a total of 360 kilowatt hours used.
Now, an all-electric house in a cold climate (eg Tas) would typically use 20,000 kilowatt hours per annum, half of that for heating and a quarter of that for hot water. Adding another 360 kWh for Christmas lights is no big deal especially when you consider that household use of electricity is only 20% of total demand anyway (varies between the states).
Even if you compare it to a typical house in a warmer climate (eg Adelaide) using 5000 kWh per annum, it's only a 7% or so increase and then only for a very small % of houses.
So, we're adding maybe 5% to the power demand of perhaps 2% of houses with significant scale Christmas lights displays. That's 0.1% of domestic electricity or 0.02% of total electricity use. In other words, if we got rid of Christmas lights it would offset growth in power demand for all of 3 days. Not a big saving but one which would disappoint a lot of young kids to achieve. It would be better, and far more effective, to just switch every office computer off at night instead or stop leaving the TV on standby.
Obviously there are individual houses using lots of power for light displays but it's not a big deal overall. Even my own display, which is understood to be the highest powered private house display in the state and is quite well known (a few hundred visitors tonight - there are quite a few people standing in the driveway right now), uses less power each year than would be saved by putting in one domestic solar hot water heater. (Peak power 18.7 KW, average load during operation 11 KW).
The attached graph shows Victorian power demand (had to pick somewhere and Vic is larger than Tas) for the past 24 hours. Not much of a spike when the lights go on. The 2 spikes in the early hours of the morning are due to off-peak hot water switching on. Note the scale doesn't start at zero. Green line is demand for power with scale on the right. Red line is spot price with scale on the left. Note that the time is standard time NOT daylight savings time. Source of the chart - NEMMCO (National Electricity Market Management Company).