All five alleged 9/11 plotters skipped Friday’s legal arguments framing their eventual death-penalty tribunal, voluntarily staying behind in a secret prison lockup.

Army Col. James Pohl, the tribunal judge, earlier this week ruled that accused mastermind Khalid Sheik Mohammed and his four alleged accomplices could voluntarily skip portions of this week’s hearings.

Throughout the week, some of the accused, others did not. Mohammed, 47, at times watched the proceedings on a 40-second delay video stream between the maximum-security war court and a nearby cell.

But Friday was the first day that all five did not come to court at the same time.

Friday is Islam’s holy day, akin to the Sabbath when many Muslims typically take the day off for Friday prayers.

Earlier this year, Pohl agreed to postpone hearings until after Ramadan, Islam’s holy month of fasting and prayer. But he rejected a request from defense lawyers for Mohammed and the four men that he not hold hearings on Fridays.

Pohl held the Sept. 11 arraignment on a Saturday, May 5, and has planned to hold a Sunday session in the case that was scrubbed for Tropical Storm Isaac in August.

With all five men absent, the defense lawyer for alleged Mohammed deputy Walid bin Attash chose to forgo the traditional black cloak she has steadfastly worn to court. Chicago attorney Cheryl Bormann, now on the Pentagon payroll to defend Bin Attash, 34, caused a stir in May by appearing in court in a traditional abaya and hijab — black scarf and gown.

The five men are accused of organizing, financing and arranging travel for the 19 men who hijacked aircraft on Sept. 11, 2001, then crashed them in New York, Pennsylvania and at the Pentagon, killing nearly 3,000 people. They got to Guantánamo in 2006 from up-to four years of clandestine CIA detention and interrogation, and eventually got lawyers.