Like many people who were in any proximity to the events of Sept. 11, the artist Spencer Finch often thought later about the color of the sky that day, the kind of crystalline blue that pilots and meteorologists call “severe clear.”

And when he was chosen more than two years ago to create the only work of art commissioned for the National September 11 Memorial Museum, he knew that the sky — or more accurately its continued existence in collective memory — would be his subject. The problem was how to find a way to get at something so evanescent and powerfully evocative. “It had to be believable,” he said, explaining what believable meant to him in such a case: “It had to be about that human quality of remembering, how it’s so fuzzy in some ways, and in other ways it’s so completely clear.”

When the museum opens this week to survivors and family members of Sept. 11 victims, and on Wednesday to the public, Mr. Finch’s solution will be put to the test in a kind of consecrated and highly contested space that would put any work of art under intense pressure to play a meaningful role. “Trying To Remember the Color of the Sky on That September Morning” is a monumental but at the same time delicate work made up of 2,983 individual squares of Fabriano Italian paper — one square for every person killed in the Sept. 11 attacks and in the 1993 bombing of the World Trade Center — each hand-painted a different shade of blue by Mr. Finch. He said he likes to think of them as drawings, and he has arranged them in a wide grid that towers almost 40 feet high, covering most of a central wall at bedrock level, behind which lies the repository, closed to the public, for unidentified remains of those who died at the World Trade Center.