I'll be visiting Xi'an on the first week of October this year (2014) and I am planning to hike up Mount Hua. I've read that it's generally cool during October period. Would wearing a sweater be suffice once you hike up the mountain or should I be wearing something thicker?
Doing a google search for "Mount Hua weather" I found this page which states that the average temperature is 6.8 degrees. So something warm that also protects you from the wind will be important. They also rent out overcoats on the mountain. I would assume that in perfect conditions, you do not need a lot, but the moment it's overcast and windy you will have an issue with only a sweater. Looking at the photos online from people climbing it I would suggest to bring something more than a sweater.
They also say that you should avoid the first week of October:
Please do remember to avoid climbing on holidays of China, such as the May Day (May 1st to 3rd) and the National Day (Oct 1st to 7th).
From my experience, this is a smart advice, and I can only recommend you to follow it. You can find several different reports from reputable sources only form last year (SCMP, IBT), and if you take a look at the photos, you will know what to expect. So again, if you planned to go almost anywhere touristy in China during the first week of October, don't do it.
Last year there were more than 27,000 tourists on the mountain on one day, but its capacity was no more than 20,000. Waiting times for the cable car were more than 4 hours. 10'000 people had to stay on the mountain over night because they cable car down was so overcrowded.
These pictures were taken on the mountain during golden week: