In the Northern Hemisphere, September signals the start of autumn, associated with cooler weather and foliage. The maple leaves turn red every September.
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Academic Beginnings
Many educational institutions start their academic year in September, particularly in Western countries. I'm starting college this September.
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Historical Events
September can reference historical events; knowing context is important, like in 'September 11' for 9/11 attacks. Documentaries about September 11 are somber to watch.