From the AP:
President Barack Obama is slated to speak at the Democratic National Convention on Wednesday, July 27.
That's according to two sources familiar with the plans. The sources requested anonymity because they had not been authorized to speak about the schedule.
Obama's address to Democrats will come on the next-to-last night of the nominating convention in Philadelphia.
Massachusetts Sen. Elizabeth Warren has been invited to speak on the first night of the convention.
Normally the VP speaks Wednesday, as well, although in 2012 with a shortened week due to the Labor Day holiday, VP Biden spoke on Thursday. In 2008, which is the better comparison, Obama’s former rival Hillary spoke on Tuesday while former President Bill Clinton spoke on Wednesday right before VP Biden spoke, and Obama made a short appearance on stage with Biden, so they appeared together. My guess is that is the model Democrats will follow again this year, but I could be wrong. Since Obama is President it is possible he could speak after the VP candidate but unlikely.
UPDATE - Politico says Sanders aides are betting on either Monday or Tuesday night:
The most immediate question for Sanders’ aides is the uncertainty surrounding the timing of his convention speech — Monday or Tuesday of convention week — leaving them looking at precedents of previous runner-up convention appearances (Clinton spoke on Tuesday eight years ago).
Splitting time with Warren does not seem likely to me, so I still think Tuesday is the most likely night.