Adding outdoor lights can add to the spirit of Christmas or the festival time. If you observe the basic precautions, it can be safe too. Lighting can be used to create a dramatic effect, provide an interesting visual backdrop to some design or foliage or simply add some extra dimension to the festivity. These days small ornamental string of lights are available with chip-on-board type controllers to add interesting light chasing effects. But most of them are intended for in house decorations and lack any serious water-proofing. Besides the quality of insulation is not in consonance with the outdoor use.
When you use any electrical item outside your house, check that it is designed for outside use specifically. This applies to not just the string of bulbs, but also the extension cord and any control gears. Allow extra length of cable to rig up your lighting and make sure that the cable is securely tied to some physical entities which do not sway in the wind. It is equally essential that the cable is not left around on the ground. The mains feed is preferably taken through a EARTH LEAKAGE CIRCUIT BREAKER (ELCB) which can switch off the supply if there is any leakage to the ground through humans, animals or other objects. The current to trigger this can be so low that the risk of electrocution is minimized.
If you need to connect some additional lighting in parallel to the one already present, check the rating of the mains cord that will supply both lighting. It must be rated to carry the additional load safely. Most of the time, the thin wire is rarely rated to carry more than its load - about a few Watts. It would be prudent to rig up a thicker cable designed to carry about 4 times the expected load from the beginning. This would provide the required margin if you add more lights. If you are not too sure about the ratings of the cable, read the label or packaging.
Check the lighting carefully for frayed wires or exposed wires and broken sockets before you deploy them. It is not adequate to wrap insulation tapes on the exposed wires for outside use. Moisture, or water can seep in between the fold of the insulation tape and might pose a risk of electrocution later.