The Frederick Law Olmsted National Historic Site is hosting a guided stroll through the Fens in Boston’s Back Bay followed by an optional tour of Fenway Park next month. The guided walk on June 2 at 10 a.m. will examine the history of the Fens, focusing on its transformation from marsh and muddy flats into an urban oasis as the first link in Olmsted’s Emerald Necklace of parks. After the two-hour walk, participants may take a tour of Fenway Park starting at 12:30 p.m. Space is limited for both events, and reservations are required; call the Olmsted site at 617-566-1689, ext. 216. For more information, visit www.nps.gov/frla.