I am planning to travel from Shenzhen to Yangshuo on Friday Jan 20th.

Helen, I guess you plan to go to YS by bus. You can find many bus stations in Shenzhen that have schedules to YS every day. It is suggested you take a bus from Bao'an Station or Yinhu Station as the two stations have more schedules than others.
Bao'an Bus Station: 7:20, 10:00, 18:50, 19:30, 21:50
Yinhu Bus Station: 7:50, 10:30, 16:00, 18:20, 21:20.
The booking website doesn't have English version. It is advised you go to the bus station early in the morning to buy the ticket.

THANKS SO much ,,, FINGERS crossed they haven't sold out... i didn;t realise how busy it would get (for new year). Do you have any recommendations of other destinations to get to from Shenzhen?
THANK YOU.

For other transportation option, you are advised to take high-speed train from Shenzhen North Railway Station and get to Guilin North North Railway Station first. You can ask help for TravelChinaGuide. The provides online ticket booking service. Their e-mail is bookingtravelchinaguide.com. After arriving, you can find several direct buses to the destination. I remember that the bus schedules are 10:30, 12:30, 14:00, 15:30, 16:30, 17:30, 18:30 and 20:00. The trip takes around 2 hours. And the ticket fare is CNY 50 per person. Hope helpful!