Small square (square??) 'snowflakes' will start falling down the monitor and the screen will slowly go all frosty (like a steamy window).
And just like a steamy window, you can draw on it - use the left button to draw on the screen - perhaps a love-heart for your beau. Or even a selection of naughty and nice friends.
Once done, click the 'defrost' button to clear the screen.
In typically cryptic fashion, Google announced the feature in somewhat poetic form:
Through the fog, you have to peer;
Because it's the most wonderful time of the year;
Your page turned into a winter wonderland;
When you typed in that search command;
You can always defrost the window;
Or just let it snow, let it snow, let it snow!
It appears that the 'trick' will work only with browsers that support the "canvas" element of HTML5. This appears to be available with IE9, Chrome (obviously!) and Firefox 8.0.1 or later. This author found it would not work with IE7.
The let it snow egg is the latest in a long history of trickery wrought by Google upon its users. Recently we had do a barrel roll, then there was Gravity (which seems to no longer work), askew which does exactly what it claims, recursion which asks us if we really meant 'recursion;' the loneliest number - it's 1 of course and the answer to life, the universe and everything.