Gary Gambrell stood silently and respectfully as he watched the changing of the guard at the Tomb of the Unknown Soldier in Arlington National Cemetery. The all-volunteer unit of sentinels which guards the tomb 24 hours a day, 365 days a year, in all weather conditions perform the elaborate ritual every half hour from April 1 through Sept. 30 and every hour from Oct. 1 to March 31, according to the Arlington National Cemetery website. But on this day, a slight variation occurred.