hard cold starting silverado!

I have a 2000 silverado 5.3. The problem is that when it gets cold below 32 degrees it becomes very hard to start. Cranks for quite a while before it finally turns over. Only on the coldest day. If I drive on those days and start it every 4 hours or so its fine. If I leave it about 8 hrs. I am going to have a problem.

Numerous things can cause a hard start condition.
First, what grade of fuel are you using? Any thing more than 87 octane can cause difficulty starting due to the faster evaporation rate of the higher octane.
Second, There have been numerous hard start problems caused by loss of fuel pressure. Fuel pressure regulators seem to be the biggest culprit.
remove the vacum line to the regulator and see if there is any evidence of fuel in the vacum line.
Also if you have access to a fuel pressure gauge, you can check the fuel pressure and if there is any bleed down.
Third, throttle bodys on those trucks have been know to cause low idle, stall and difficult starts from a build-up on the bore and blade...a good cleaning of the throttle body will help that situation.
Last, The possibility of a schewed sensor reading that is temperature sensitive could also be an issue...you will need a scan tool for that.
Hopefully this will lead you in the right direction.