As mentioned elsewhere here at KN@PPSTER, I expected US vice-president Joe Biden to announce his candidacy for the Democratic Party's 2016 presidential nomination no later than mid-September. I still expect him to jump in, and so do others (as you'll find if you do a little Googling or keep an eye on memeorandum).
It just occurred to me why he may be waiting until "the last minute" (the end of this month or, at latest, early October):
Biden is described as a "devout Catholic," and he and his wife were on hand to greet Pope Francis, in their capacities as vice-president and "second lady," when he arrived for his visit this week. Since the vice-president also presides over the US Senate, Biden will presumably be a prominent figure as the Pope addresses a joint session of Congress (the first time a Pope has ever done so).
If he had announced his candidacy already, he would likely be seen as exploiting the Holy Father's visit as a "campaign event." Similarly, he's unlikely to announce his candidacy while Pope Francis is still in the US, because that would look like he was using the Pope as a campaign kickoff prop. He would offend Catholics who felt like he was using them, and he would offend anti-Catholics who don't like the idea of Rome playing a part in US politics.
On the other hand, if he waits until the Pope leaves, he will largely avoid those two pitfalls AND he will be announcing right after a week in which he's been unusually prominent (for and as a veep) BECAUSE of the papal visit.