The idea is to have a dedicated day that would be widely recognized as the one on which you should try your best not to consume any psychoactive substances.
To encourage healthy lifestyle in a playful way through globally recognized practical challenge.
To clean the organism from psychoactive substances at least for a day.
Practical side: The spectrum of psychoactive substances range from hard drugs like heroin and crack on one side to "soft" ones like caffeine and sugar on the other. On the "soft" end it becomes hard to define the border between psychoactive and non-psychoactive, therefore such initiative shouldn't encourage overthinking, its aim would simply be to motivate people to give up their routinely used psychoactive substances for a day.
So if you're say a regular coffee drinker, you'd have to give it up on that day, if you like beer a lot and the day happened to be on weekend, you'd have a challenge to skip drinking. Any attempt to give up any of the psychoactive substances or lower their consumption on that day would be welcomed with the basic aim being not to consume any of those at all for 24 hours.
If such a day received enough attention/support, a dedicated database containing information on all known psychoactive substances could be linked to the official drug-free day website so that people could check whether some particular substance/product qualifies as being psychoactive or not, but again - this shouldn't be turned into a mania, avoiding substances that are generally regarded as psychoactive would be enough to fulfill the goal of the day.
The novelty of the idea: There are at least a few related initiatives aiming to reduce drug use/raise awareness about the topic, but they're all mostly about the spread of information on drug harm and not practical challenges. As far as I know, there are no such widely recognized day encouraging to give up psychoactive substances for 24 hours.
Why just one day: While not consuming psychoactive substances for just one day might seem like simply playing around without any tangible benefits, marketing-wise one day is a good period of time if the aim is to get as many people as possible involved in the challenge and make such day globally recognized.
One drug-free day is a feasible goal for almost everybody. Attracting masses of people to participate in the initiative for a short time arguably will have a stronger positive cultural effect than having a small group of people participating in longer-lasting challenges. Even though a rather utopic aim, the idea of having the whole world totally sober for a day is attractive and hence encouraging.