Simply designating a villain schedule on the forums won't get you very far, because the vast majority of the player base doesn't read them. However, it has also been my observation that the vast majority of players are loathe to take their own initiative - you can generate interest just by providing an activity that they can join.
So, if you want to start a designated villain day, start by running your own villain teams. When you advertise in LFG, make sure you mention that it's a weekly thing (or whatever timeframe you decide). It might start out slow, but people will show up. The most important thing is to break the "nobody plays redside" negative feedback loop by doing it in a way that is highly visible, but you also want a regular schedule so people associate that time with redside activity.
This isn't to discount VileTerror's post about recruiting in global channels, of course. My point is simply that putting together a regular community event (which is essentially what this is) takes some legwork by someone (or several someones) willing to take the lead. Picking a day to "ensur[e] high pop" isn't enough.