Donald Trump says the fall debate schedule is "unacceptable," raising the specter that he may try to skip them.

In a tweet on Friday night, Trump incorrectly said that Hillary Clinton and the Democrats are "trying to rig the debates."

In fact, the fall debate schedule was determined almost a year ago by the nonpartisan Commission on Presidential Debates, a private group made up of both Republicans and Democrats.

His primary complaint is that two of the debates are scheduled on the same nights as NFL games.

That's true. (It was also true in 2012, and the debates were still high-rated.)

In an interview with ABC News, Trump said he's "fine" with the commission's three debates, but objected to the specific dates.

"I'll tell you what I don't like. It's against two NFL games," he said. "I got a letter from the NFL saying, 'This is ridiculous.'"

An NFL spokesman said Saturday: "While we'd obviously wish the debate commission could find another night, we did not send a letter to Trump."

On Sunday morning, a Trump aide said that "Mr. Trump was made aware of the conflicting dates by a source close to the league."

"It's unfortunate that millions of voters will be disenfranchised by these chosen dates," the aide added.

The debate organizers have a different view: They note that the debates are easy to find all across TV, and people who choose to watch the football games live can still watch the debates later through video on demand.

Because Trump skipped one of the GOP primary debates in January, there has been speculation among media types that he may quibble with the fall debate schedule or even threaten to not show up.

Campaigns routinely haggle over the details of the debates, right down to the temperature of the studio, but every major party candidate in modern times has ultimately agreed to participate.

It would be risky for Trump to turn down the debates. Voters consistently tell pollsters that the sessions help them decide which candidate to support.

Trump hasn't threatened a boycott. He told ABC, "I like three debates. I think that's fine. I think it's enough." (The ABC interview was taped Friday.)

But he focused on the NFL scheduling conflict. RNC chief strategist Sean Spicer seconded it in an interview on CNN Saturday morning, and added, "The entire system needs to be re-looked at."

The commission responded to Trump on Saturday afternoon by saying that it "announced the number, dates and sites for the 2016 general election debates in September 2015."

"The CPD did not consult with any political parties or campaigns in making these decisions," the group said.

According to commission officials, sporting events, religious holidays, and other factors cause scheduling headaches every four years. The group seeks to space out the debates and schedule them for different nights of the week.

The first debate of 2016, slated for September 26, coincides with "Monday Night Football" on ESPN.

The vice presidential debate is scheduled one week later on October 4, a Tuesday.

The second presidential debate, on October 9, coincides with "Sunday Night Football" on NBC.

The final debate is slated for October 19, a Wednesday.

NFL games are played on Sundays, Mondays and Thursdays in the fall. Fridays and Saturdays are effectively ruled out because TV viewing is lower on weekend evenings. That only leaves Tuesdays and Wednesdays.

One complication was Yom Kippur on Tuesday, October 11 and Wednesday, October 12.

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Trump's tweet also brought up Bernie Sanders, saying "same as last time w/ Bernie," a veiled reference to the Democratic primary debate calendar. Many critics believed that the Democratic National Committee sought to minimize the debate schedule, thereby favoring Clinton over Sanders.

The Commission on Presidential Debates has many critics too, but it was specifically formed to provide a nonpartisan structure for debates that doesn't favor one candidate over another.

The parties are not involved in the debate scheduling, a point that Spicer made on CNN: The commission "announces the dates without consultation of the parties or the candidates."

Spicer acknowledged that the dates were set last September, before the primaries even began, but said that "doesn't mean it's a good system."

"You look at the debates being on major NFL nights and you wonder why people are upset about that? This was a dumb idea and should be revisited," Spicer said, throwing the RNC's support behind Trump's complaint.

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When Trump tweeted on Friday night, New York Times political correspondent and CNN analyst Maggie Haberman responded, "Is this a prelude to not doing all three?"

Another interpretation is that Trump wants to make sure he has the biggest possible audience for the debates.

Before Trump weighed in, former Obama speechwriter Jon Favreau asserted on Friday's "Keepin' It 1600" podcast that Clinton should call for more debate sessions.

If Trump "says no, then he looks weak, which is against brand," Favreau theorized.

His podcast co-host Dan Pfeiffer, a former Obama aide and CNN commentator, brought up the ongoing guessing game about whether Trump will participate.

"The rumor is that Trump may not agree to the debates. He is not engaged" in the normal negotiations between the campaigns and the commission, Pfeiffer said.

"I heard the same thing actually," Favreau said.

Programming note: Trump communications director Jason Miller will be interviewed on CNN's "Reliable Sources," Sunday at 11 a.m. ET.