Cafe culture for a non-drinker?

Not a problem. Bottled water is freq ordered with pastries.
Your hot choc will be xtra flavorful.

Which cafes do you plan to visit? That will determine the crowds.

Cafes would lose a lot of business if they only served coffee. Hot chocolate is totally acceptable, as is juice, water or beer.

no one cares what you drink in any European city. And Vienna rather specializes in hot chocolate with calorie bombs of whipped cream. But seriously, feel free to order water, soda, beer or wine, tea, coffee, juice, hot chocolate etc. You will see it all at cafes. Each city tends to have their specialties -- in Vienna the melange (what is called a creme in Paris) and hot chocolate are commonly ordered. In Paris, they have diabolos with flavored syrup and soda water or sprite like soda as well as cremes and various alcoholic drinks.

Hi Emily, I have no preference. I guess the two biggies: Café Central and Dremel. Unless you would like to suggest another? I am flexible. Since it is my first visit, I don't know the difference between one and another.

Try the Heiner pastry shop/café in the Wollzeile. As good as Demel, with a glorious history as a court confectioner as well, but not as expensive and probably not as crowded.

You can order anything you like to drink at the cafes. I remember having a diet coke once with my Sacher torte. Anything goes and if you don't like coffee or tea then water always works or a hot chocolate. Earlier is usually better for crowds.

My suggestions (all have very different feel):
Hawelka - iconic, cluttered, nutty professors
Sperl - what you picture in your mind when you think of a Viennese cafe
Demel – better than Sacher
Cafe Central – most beautiful cake display
Gerstner – best strudel, sour cherry in particular.
Cafe Diglas – best cakes

This is one of those things you don't have to plan -- just plop down when you come to an inviting cafe. You can see the cakes and pastries and such before you make the commitment. There are lovely cafes here and there, you don't need to seek out the tourist traps. To me it was like Angelinas in Paris -- no reason to stand in that line for a crowded touristy experience when there are literally dozens of great pastry shops all over. Same in Vienna -- so many great options. No need to plan for a particular one.

Somewhere in my research of Cafe's and pastries I saw that
Dec. 06 was Sacher Torte Day. Is this really a holiday, or just some
made up promotion? If real, anything special happening that day: a torte competition
or a discount on all Sacher Tortes?

We had Sacher cake and coffee at Cafe Diglas in Vienna and are still trying to figure out what all the fuss is about!! I found it rather dry.

Yes, Sachertorte is my least favorite of all cakes. Cakes in this part of the world are typically very dry so they can remain on the shelf for an extended amount of time. Don't expect Betty Crocker or Duncan Hines. To cut this dryness, copious amounts of creme, jam or something soft will be added into between layers. They look a lot better than they taste. I actually always order strudel, which is always good.