The first word is part of weih soft = holy, gothic Weihs, wîh Old High German, Middle High German derived important. Some language researchers * az important than passive participle to the root wîq (soft), so that it is a separate flock of sacrificial animal acting and refer to the Latin victima and his clan. 
Luther thought to weigh and formed Wygenachten, "because we weigh the child."  But the second word is part of a celebration and pointed to unusually high age. Germania Tacitus wrote in chapter 11: "nec dierum numerum, ut nos, sed noctium computant" ( "Not the number of days as we do, but the nights they are"). Etymologically is also coming from the Middle High German ze wihen seam (in the holy nights') view, which is already on the Germanic Mittwinternächte celebrated indicating time, but it is unclear whether these festivities actually winter solstice took place .
Theodor Storm formed from the noun "Christmas" then the verb weihnachten.  In his poem Knecht Ruprecht of states in the opening and closing lines:
From outside 'by Walde komm ich her;
I must tell you, Christmas is it.