For the third time in U.S. history, the House of Representatives on Wednesday will likely vote on articles of impeachment targeting the president. Even with the inclusion of that “likely,” we don’t know much more than that.

Normally, the rhythms and schedule of Congress are readily discernible. There are expected interjections and limited exposition, and one can figure out with some accuracy how long a vote or debate might take. With impeachment? Expect the unexpected.

The Washington Post’s Paul Kane knows better than most what a normal day in the Capitol looks like. He walked me through the limited number of things we know with certainty and the numerous things we don’t.

The House will open for business at 9 a.m. In other words, the actual process of holding the vote will begin at 9 a.m. and progress from there. This does not, however, mean that the final vote will occur on Wednesday morning.

The time of the final vote is uncertain. While one might expect a vote by midafternoon on a normal issue, Kane is confident that this rule of thumb won’t apply on Wednesday. It’s unlikely the final votes will occur until early evening — or later. (Hence that “likely” up there.) Democrats are currently projecting a final vote in the 6:30 to 7:30 p.m. time frame. If that’s accurate, Trump may be impeached in the middle of a campaign rally in Michigan.

Or not. When the House Judiciary Committee’s debate of the impeachment articles stretched late into the night last Thursday, the decision was made to kick a final vote to Friday morning. Such a move is unlikely on the final vote in the full House, but it’s hard to say with any precision when that vote will occur.

Before the final vote, there will be six hours of debate. After an initial vote approving the rules of the debate, House Democrats allotted six hours for debate of the articles of impeachment. But there’s an important caveat to that time limit.

Six hours can include “magic minutes” of indefinite duration. House Speaker Nancy Pelosi (D-Calif.), Minority Leader Kevin McCarthy (R-Calif.) and Majority Leader Steny H. Hoyer (D-Md.) have special privileges that allow them to use as much time as desired, with the total being counted as only one minute of the allotted five hours. Kane notes that former House speaker John A. Boehner (R-Ohio) once had an hour-long “minute” during a debate over an environmental bill in 2009. Pelosi herself packed 486 minutes into a “minute” during a debate last year.

She’s not likely to use up hours and hours of time on Wednesday — but we can reasonably expect that McCarthy’s minute will be anything but.

Republicans will try to throw wrenches in the plans. Those who’ve been watching the hearings and votes leading up to this point are no doubt familiar with the various procedural efforts that can be deployed to further gum up an already gummy process. On Wednesday, we can expect Republicans to call for a number of votes that stand no chance of passage but must be addressed before the impeachment debate can move forward.

Kane expects this will happen several times during the day, with each effort eating up at least half an hour.

Members of the House will need to watch their language. While it may not seem like it, there are clear boundaries to what members are allowed to say from the floor of the House. Members of Congress can’t be sued for libel based on something they said on the floor, thanks to the “speech and debate” clause of the Constitution, but rules of decorum — which originated with Thomas Jefferson — mandate that they not disparage the president in certain pejorative terms.

When Pelosi referred to several tweets from the president as racist during debate in July, it kicked off a dispute that eventually triggered a rebuke from Hoyer.

“It’s out of order if you question the president’s personal conduct, even if you insinuate it,” Ilona Nickels, a former congressional analyst with the Congressional Research Service and Library of Congress, told The Post in the wake of that dispute. “So references to the president in that way — members who have called him a liar or a hypocrite, or accused him of being a demagogue or unethical — that language has all been challenged successfully.”

Assuming at least one article is approved, a final vote will be held to allow Pelosi to appoint impeachment managers. Those managers (who likely won’t be identified on Wednesday) will serve as prosecutors in the president’s trial in the Senate.

Close readers of the text above will note the one point of certainty in everything Kane outlined: that everything will begin at 9 a.m. From that point on, things are likely to get hectic — and largely unpredictable.