If aliens are watching us, what better way to send them a message than to turn all lights off on the planet for a day or two? Such a message would prove that humanity has achieved a new level of maturity through global collaboration.
A turning off of the lights, not electricity, as so many emergency services rely upon it.
A response may not be received or understood for possibly decades, but that is not the point. We sent the voyager probe out with messages on it and the hopes of a response from that are even less likely. So a response is not the goal. The goal is massive collaboration on a global level to get humans of every political background to work together towards a single goal. Such an effort, if successful, would usher in a new Age of Enlightenment and prove to God that we are a species who is not dependent on technology, but rather the technology is dependent on us.
Opponents of this idea may argue that it is unwise to send a message to any alien or intelligent species that may be out there watching. However, if you honestly believe that hostile aliens exist and will attack earth, then we need to immediately cease all radio wave emissions from the planet to protect ourselves. There is no evidence to support such a claim that hostiles are watching, so living in fear of aliens should be considered a radical idea.
Instead, we should be extremely conscious of our global image and try our best to prove to any aliens that we, the humans of earth, are in charge of the technology and not vice versa. We currently live in an out of control state where technology is enslaving us. Any alien watching could easily mistake vehicles as being the most advanced species on the planet and see how they are destroying it and thus conclude we are not ready for any sort of communication.
A day of lights out, if feasible, will send the message that we are all connected, we are all one species, on one planet, in control of our own power.