Sunday, September 30
Whether (and when) Pope Benedict will use the 1962 missal has replaced expectations of the motu proprio as the center of papistical gossip. The speculation is fun, and this time the topic is not nearly so "high stakes."
Dec. 2 would be the first Sunday of Advent. Liturgically, this is significant as the 1965 Missal became mandatory on the first Sunday of Advent, those years ago. That doesn't give or take credibility from the rumor, but it is the most likely date: anything feast more significant would be riskier (like, say, Christmas), but this date is still significant within the historical context of the Latin Mass.
The recent appointment of Marini, as Marini's replacement, means that it is now actually in the realm of possibility that Benedict even could use the Extraordinary form publicly.
But, I think it still seems unlikely. We'll see!