Why it is necessary for hair combing (梳頭, shūtóu)?
Chinese believe that it will bring long and lasting marriage. So both bride and groom will have their own hair combing ceremony.
When should it be done?
It is conducted on the eve of the wedding by the women, typically the parents.
In this photo of my aunty, the ceremony is done before the groom came to fetch the bride. Both parents said the four blessings:
一梳梳到尾 (1st combing, be together till the end of the road)
二梳百年好合 (2nd combing, happiness and harmony together till old age)
三梳子孙满堂 (3rd combing, blessed with many children and grandchildren)
四梳白发齐眉 (4th combing, blessed with longevity)