It’s official: Mayor E. Denise Simmons has declared Oct. 8 “Joan Baez Day’’ in the city of Cambridge. Simmons issued a proclamation stating that Baez was a key figure in the city’s fledgling folk scene, dating back to her earliest performances at Club 47 in the late 1950s, and “she continues to remember Cambridge fondly as a hub of creativity and inspiration.’’ The timing is perfect because Baez is in town this weekend. She’s scheduled to perform at the Citi Wang Theatre on Saturday night along with special guest Mary Chapin Carpenter.