The September 1st to October 15th period tends to be cold so warm jackets - preferably down - are recommended. The upside of trekking during this time is that it tends to be dry and the air is clear making views phenomenal.
Waterproof outerwear is also required. Every trekker's personal clothing should include some of the following : wide brimmed hat(cloth/felt) balaclava/ski cap, scarf, T-shirts, Long-sleeved shirts, fleece jacket/warm jumper, fleece trousers, trekking pants, pajamas, shorts, socks/stockings, trekking shoes, light trainers, sandals, sunglasses, sunscreen, water bottle, day sacks.