This believe is based, among other, on the assumption jewelery demand increases at the end of the year.
Indeed, if you take a look at the average monthly returns over the last 41 years (1969-2010), price of gold was increasing in September, November and December:
Fig. Average monthly changes
Fig. Average daily changes in particular quarters
Fig. Distribution of monthly changes
Fig. Distribution of bi-monthly changes
Fig. Distribution of daily changes in particular months
Fig. Histogram of changes in September
Fig. Histogram of changes in November
I just can't see a visible cyclical / seasonal pattern in gold prices. But maybe somebody else can and would correct me?
To see daily chart of gold futures prices go to: http://www.wolframalpha.com/input/?i=gold&a=*C.gold-_*Commodity-
You can also find my bookmarks for articles about gold at: http://delicious.com/reakkt/gold