Anna
Have a look at the websites of a couple of forum members who work in the travel/tourism industry in Guizhou. It might give you some ideas even if you don't want to formally sign up with them.
See Tony Chen (who is a Miao and has articles on the main tribaltextiles website)
http://www.tribaltextiles.info/articles/Tony_Chen.html Any one of his articles has a link to his website on the bottom.
See also Wang Jun's website
http://www.gzcyts.com/en1/ Wang Jun is a national guide who has worked with Gina Corrigan and arranged many tours for her - I have been on 3 of them and he is first rate. The guides with GZCYTS can all give individual support. I think that both websites I mention have a list of festivals, the dates of which, change year by year.
You might want to have a look at my China page on the tti website to give yourself a flavour
http://www.tribaltextiles.info/Countries/China.htm and also see my diaries for my trips. Of course, everything changes so fast in China that all the individual details will have changed now.
Good luck!
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Pamela
http://www.tribaltextiles.infoon-line tribal textiles resource