This Idea will work best if the wind is not blowing too hard. Some wind should be tolerable, though.
Start with a fountain designed to shoot water upward to a fair height. Modify the nozzle so that instead of being typically circular, it is wide and narrow, say 3 meters wide and 1 centimeter narrow.
When activated, a huge "sheet" of water is shot upwards, instead of the typical rod-shaped stream.
With nozzle dimensions of 3 meters and 1 centimeter, quite a lot of water will be involved. A rather large pump will be needed. Also, that 1-cm dimension is related to the maximum height that this fountain can reach; if it was only 1 millimeter the water would break up into droplets due to air resistance at a much lower height. And since we are talking "advertising" here, we obviously want as high a fountain as we can get.
Next, we arrange so that the water is not shot QUITE straight up into the air. We want a small angle from the vertical, so that when the water reaches maximum height and starts to fall, it will not interfere with the rising sheet.
Next, we add some appropriate non-toxic substance to the water, to turn it milky-colored. This is more likely to be a suspension of fine particles than it is to be something actually dissolved in the water, but whatever can work is acceptable to this Idea.
So, when this fountain is running, we have a sheet of water reaching to a considerable height, which breaks up into droplets and arcs downward back into the pool/reservoir from which the fountain recirculates it. Note that the "sheet" aspect will be maintained on the downfalling side, even if it consists of many many small droplets.
Now we wait for night, and turn on two projectors, one on each side of the tall arcing sheet of water. Since the water is milky, it is quite reflective and also is mostly opaque. Two separate tall skinny (3 meters would be skinny in comparison to the fountain's height) images can be projected onto the milky water sheets. All night long you can be advertising, for example, a word made of large letters positioned vertically. For example:
I'm assuming we can't make the projections bright enough to compete with the Sun in the daytime. That's OK, because in the daytime people would rather see the scenery than huge advertising signs --and this one disappears whenever you turn the power off!