Gold is processed in the jewelry industry. Companies in this industry do not store a lot of raw material. They buy their raw material - gold - on Friday for delivery on the weekend or Monday morning. Their production units have enough gold to work during the week, thus avoiding expensive weekend work. Gold also serves as a "safe haven" for many investors. It is "insurance" against a crash or other difficult to calculate risks over the weekend. These investors also buy gold to hold it over the weekend.
Both factors contribute to many parties buying gold on Fridays. Buying gold on Friday is therefore statistically interesting.