Go to your Blogger template's Layout and open Edit HTML .
Now search for html head tag and place this code that are just going to download!!
Save and preview the changes .
You can customize the number,color and speed of the snow flakes by changing the JavaScript used with the above code ( given in bold) .
var num = 60; //controls the number of flakes
var timer = 20;//controls the speed of flakes
Wish you a very happy Christmas and prosperous New year . Do share your feedback's as comments below.