According to the Green Party’s webpage, the 2016 presidential convention will likely be August 6-7. This is the latest the Green Party has ever held a presidential convention. In the past it has always been in June or July.
The 2016 Democratic national convention will be July 25-28.
How many petitioning deadlines will they miss with this late convention date?
Very few, because they can use stand-ins in most states in which a candidate must be shown on the petition. Alabama and New Hampshire won’t permit stand-ins for president, but Greens in those states are always free to assume that Stein will be the nominee, and to sue if she turns out not to be the nominee.
Another possibility–Hillary Clinton wins Democratic nomination, and a large number of Green try to get Bernie Sanders as the 2016 nominee.
Are there certain reasons or advantages why they would wait so long to nominate rather than do it sooner?
Primary season matching funds ends when the party nominates someone. So the later in the year of the convention, the longer the period to raise money to qualify for primary season matching funds.
Also just in case something happens at the major party conventions that couldn’t have been expected, s party with a late convention would have the flexibility to take advantage of whatever surprising thing happens.