Thinking it over, I prefer option three, especially since it coincides with the CR meme. It's easier to go "yes, and I threw something up just now- here" while everyone is more excited about the prompts having just come out. The timing just works better since it's at the middle of the month, when I notice things start to slow down activity wise so it might keep the momentum going. I think it also just works well for the newer characters who might have missed out on the mingle or if you're having a late start to the month in general.

I initially liked option one more, since there have been times when I thought it'd be nice to be able to have a mingle using TDM prompts for characters already in the game. I wouldn't want to see the TDM get less activity because of it though and having it at the end of the month seems like it might be too close to the beginning of the month mingle.