Complex sentence: Although San Diego is the birthplace of Fleet Week, the event was held only that one time in 1935, and not again until 1981 when Fleet Week was revived in San Francisco in conjunction with Columbus Day celebrations.
Split into simpler sentences: Although San Diego is the birthplace of Fleet Week, the event was held only that one time in 1935. In 1981, Fleet Week was revived by San Francisco in conjunction with Columbus Day celebrations.