The letters O.S. appearing after dates (as on Thomas Jefferson's tombstone and his epitaph for his wife, Martha Wayles Skelton Jefferson) stand for "Old Style." The Julian or Old Style calendar was in effect in England and her colonies until 1752, when the Gregorian or New Style calend
Monticello will mark mark the University of Virginia's Founder's Day and the 271st anniversary of Thomas Jefferson's birth with ceremonies on the grounds. A guest Thomas Jefferson Foundation Medal-winner will speak at a special ceremony on the West Lawn at 10 a.m. on Friday, April 11, 2014 (two days before Jefferson's actual birthday).