Blanketing is one of those things that horse owners love to disagree about.  Some never blanket for any weather; some blanket all the time, from nose to tail so that the horse won't grow a winter coat (or because the horse has been clipped).  I fall somewhere in between: I blanket when it snows and when the temperatures fall below 20 degrees, which means about half the nights in the winter in Denver, plus the occasional day when it doesn't get above 20 or when we get snow throughout most of the day.

When and how often you blanket depends on many factors.  If you clip your horse so that he doesn't sweat as much when being ridden in the winter, for instance, you will need to blanket much more than if you leave him fuzzy.  (Keep in mind, however, that if you leave him fuzzy, you will need to cool him down and dry him off after riding.  Putting away a sweaty horse in freezing temperatures is very bad.)

Another factor to keep in mind is how much you plan to ride throughout the winter.  Even if you don't decide to clip, blanketing more often will prevent your horse from getting as fuzzy, which is nice if you ride a lot because then he won't sweat as much.

Finally, don't forget to consider your horse's individual preferences and cold tolerances.  One of my horses has always gotten cold very easily -- he is part Arab and doesn't get as thick or long a winter coat as some horses.  As a result he is always first into the shelter when it gets cold or starts to snow, and is quick to start shivering when he gets wet and chilled.  He would survive it, sure, but in the same way I would survive living in an igloo in the winter: not very happily.  So I blanket him when I know he'll get either cold or wet.

The other horse, by the way, cares much less about whether he gets wet, and has a much thicker winter coat -- but he gets blanketed anyway, because I figure while I'm blanketing one, I might as well do both!

The one important thing to remember is that if you start the winter with a blanketing regimen in mind, you have to keep it up for the rest of the season.  You can't start out blanketing, and halfway through the winter decide to stop, because your horse won't have built up a thick enough coat to suddenly and unexpectedly withstand the temperatures on his own.