Complex sentence: The Amazonas Philharmonic is the official orchestra of the Festival Amazonas de Ópera (which is held every year from March until May), in 2011 completing their 15th consecutive year of the festival.
Split into simpler sentences: The Amazonas Philharmonic is the official orchestra of the Festival Amazonas de Ópera (which is held every year from March until May). In 2011 the orchestra completed their 15th consecutive year of the festival.