What's a "wildcard?" Well, often when playing a card game a wildcard takes the place of another card. The value is determined ahead of time or by the player who plays it. In the same way, in the card game Uno, a "Wild" card allows the player playing it to choose the color. "Wild" equals choice in place of set, formal rules.
So, applying the same logic here, "wildcard" means no set rules today. Wednesdays are wild! Woohoo. I get to discuss whatever I want. "Whatever I want to discuss Wednesdays." I guess that would have worked too but it wouldn't have flowed as smoothly.