It's a diaspora thing, two nights because 1600 years ago it was hard to know when the new moon would have been observed in Jerusalem and difficult to get the official observation out. Presupposing that an official observation was necessary. So Obama's seder wouldn't be tonight if he kept to his tradition; he'd be holding his Saturday night, on the second opportunity.
He didn't keep the tradition in 2016, when the 'seder' was may have been billed as a seder but held on night 6 of Passover because he wasn't available the first two nights and he probably didn't know about the actual 2nd passover, the pesach sh'ni, that's allowed for. But that sort of means the seder is the meal, whenever it's held, including the lines about blessing the holy day and that this was the night that the firstborn of Egypt were slain--sort of killing off a bunch of corpses. (I also means "Hey, I held a seder Christmas morning!" is then a licit statement.)
My seder-like meal will be tonight. I don't see a reason for doubling the high days. And if I'd missed the passover, I wouldn't hold it next week--I'd wait for the pesach sh'ni.
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