is it available transportation for tourists from Beijing directly to Mutianyu on Jan 25 to 26, 2013?

There is no direct bus running at that period from the downtown area to Mutianyu. You can hire a cab to go there directly. Or you can take the bus 916(express line) from Dongzhimen to Huairou Bus Station first, get off at Qingchun Lu Beikou, Huairou Bei Dajie or the terminus and then hire a mini-bus to Mutianyu. Bus 916 can be found in the bus stop close to Exit B of Dongzhimen subway station.