The injury pattern
Falls from ladders and roofs spike between Thanksgiving and New Year. The typical case is an adult on an extension ladder that is leaning at the wrong angle or standing on soft ground, reaching sideways to clip a strand, and going over. The second most common is someone walking a tile roof that has dew on it in the early evening.
Broken wrists, broken ankles, head injuries. It is not a joke, and I have seen the aftermath at customers' houses.
Ladder basics
Set the base one foot out for every four feet of height, on firm level ground, with the top extending well past the gutter so you have something to hold. Never lean sideways; move the ladder. Have a second person foot the ladder and hand you the strands. Wear shoes with grip, not slippers.
Do not lean the ladder on the gutter itself; gutters bend and ladders slide.
Roof basics
If you must go onto the roof, do it in daylight, dry, in soft-soled shoes, and stay off tile entirely. Clay and concrete tile crack under foot and become slick with any moisture. Composition shingle is walkable when dry and warm but loses granules under repeated foot traffic.
Wet weather, wind above a breeze, or any frost, and the job waits for another day.
Electrical
Use outdoor-rated strands and extension cords, plug into an outlet with a ground-fault interrupter, and do not daisy-chain more strands than the packaging allows. Frayed cords and cracked sockets go in the bin, not on the roof. Keep connections off the ground where water pools.
A tripped breaker is an inconvenience; a cord fire in dry December brush is something else.
Or let someone else do it
There are outfits that hang lights professionally and take them down in January, and for a two-story house with tile that is money well spent. If you do it yourself, read the two companion posts on installation method and on protecting the roof while you do it.
The best decoration is one that does not end with a trip to the hospital or a call to me for broken tiles.
Roof damage is the other cost
Beyond the injuries, the other December casualty is the roof itself: cracked eave tiles from careless feet, torn shingles from staples, and bent gutters from ladders leaned on them. Each of those becomes a leak in the winter rains, so the way you hang the lights matters to the house as well as to you.