On Presidents’ Day Weekend, the president was nowhere to be found on campus.

With resignation rumors swirling, University President Lawrence H. Summers cancelled two appearances that had been scheduled for Thursday, and he was in absentia Friday when Harvard unveiled its most extensive Allston expansion plans to date.

On Friday, University officials unveiled key details of the planned Allston campus—including an announcement that German architect Stefan Behnisch would be the lead designer for a 500,000-square-foot science complex that will house the Harvard Stem Cell Institute.

But with Summers off in Utah, the president did not join Boston Mayor Thomas M. Menino at a Friday morning speech in which the mayor trumpeted Harvard’s Allston plans.

That wasn’t the first time last week Summers was missing in action.

Early Thursday afternoon, Summers skipped a scheduled session with Institute of Politics fellows who just recently had arrived at Harvard for a semester of study.

“We were told he had a meeting that ran long,” one Institute fellow wrote in an e-mail to The Crimson.

Summers had also been scheduled to meet with The Crimson’s editorial board Thursday evening, but his spokesman, John D. Longbrake, cancelled that session early last week.

Longbrake said that the president left campus Thursday afternoon for a ski vacation with his children, and that Summers’ staff has known about the ski-vacation plans since last month.

Summers has two teenage daughters and a son from his first marriage. In December, he married Elisa New, a professor of English at Harvard, who has three daughters.

When asked why Summers did not cancel his appointment with The Crimson’s editorial board farther in advance, Longbrake said that the president was not at fault.

“I made the mistake,” Longbrake said.

NOTHING ‘EVIL’ HERE

In a conference call regarding the Allston plans Friday, Harvard’s second highest-ranking official, Provost Steven E. Hyman, also said that Summers’ family vacation was “long-planned.” Hyman added that “the timing of this announcement was dictated by the availability of the mayor.”

And in an interview Friday, Professor in Practice of Urban Design Alex Krieger, whose firm is involved in the campus expansion project, said of the Allston announcement: “This has been scheduled for this week for quite awhile.”

Several elements of the expansion plans had been known for months, but the University did not reveal them publicly until Friday. Still, Krieger cautioned against reading too much into the timing. “I don’t think there’s anything ‘evil’ here,” Krieger said.

Harvard spokeswoman Lauren Marshall wrote in an e-mail Sunday evening: “The development of Allston is an important partnership between the city of Boston and Harvard, and we were delighted with the Mayor’s decision to highlight and embrace Harvard’s Allston announcement in his speech before 1000 city leaders, which dictated the timing of the announcement.”

Longbrake, Summers’ spokesman, said the president is expected to be back at Harvard today. And Longbrake confirmed that Summers still plans to speak to undergraduates at Dunster House at 9 p.m. tomorrow.

—Daniel J. Hemel contributed to the reporting of this story. —Javier C. Hernandez contributed to the reporting of this story. —Natalie I. Sherman contributed to the reporting of this story.